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The Impact of End-Tethered Polyhedral Nanoparticles on the Mobility of Poly(dimethylsiloxane) Terence Cosgrove, Steven Swier, Randall Gene Schmidt, Sairoong Muangpil, Youssef Espidel, Peter Charles Griffiths, and Stuart W Prescott Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01211 • Publication Date (Web): 01 Jul 2015 Downloaded from http://pubs.acs.org on July 10, 2015

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The Impact of End-Tethered Polyhedral Nanoparticles on the Mobility of Poly(dimethylsiloxane) Terence Cosgrove,∗,† Steven Swier,‡ Randall Schmidt,‡ Sairoong Muangpil,¶ Youssef Espidel,† Peter Griffiths,§ and Stuart Prescott∥ School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK, Dow Corning Corporation, Midland, Michigan 48686-0994, USA, Program in Chemistry, Faculty of Science, Maejo University, Chiang Mai 50290, Thailand, The School of Science, University of Greenwich, Medway Campus, Central Avenue, Chatham Maritime, Kent, ME4 4TB, UK, and School of Chemical Engineering, UNSW, Australia E-mail: [email protected]

Abstract A series of dumbbell shape nanocomposite materials of polydimethylsiloxanes (PDMS) and polyhedral oligomeric silsesquioxanes (POSS) was synthesized through hydrosilylation reactions of allyl- and vinyl-POSS and hydride-terminated PDMS. The chemical structures of the dumbbell shaped materials, so called POSS-PDMS-POSS triblocks, were characterized by 1H

NMR and FT-IR spectroscopy. The molecular weights of the triblock polymers were de-

termined by gel permeation chromatography (GPC). Their size was analysed by small-angle ∗ To

whom correspondence should be addressed of Bristol ‡ Dow Corning Corporation ¶ Maejo University § University of Greenwich ∥ UNSW † University

1 ACS Paragon Plus Environment

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neutron scattering (SANS) and pulsed-field gradient stimulated echo (PFG STE) NMR experiments. The impact of POSS on the molecular mobility of the PDMS middle chain was observed by using 1 H spin-spin (T2 ) relaxation NMR. In contrast to the PDMS melts, the triblocks showed an increase in mobility with increasing molecular weight over the range studied due to the reduced relative concentration of constraints imposed by the end-tethered nanoparticles. The triblock systems were used to compare the impact of tethered nanoparticles on the mobility of the linear component compared to the mobility of the polymer in conventional blended nanocomposites. The tethered nanoparticles were found to provide more reinforcement than physically dispersed particles especially at high molecular weights (low particle concentration). The physical blends showed an apparent percolation threshold behavior.

Introduction Polymer nanocomposites have received considerable attention worldwide from research groups 1 because they offer an opportunity to modify the thermal, electrical, optical and mechanical properties of polymeric materials for use in a wide variety of practical applications. 2 Polymer nanocomposite materials based on the incorporation of polyhedral oligomeric silsesquioxane (POSS) are one of the composite materials that have drawn much interest. 3 Inclusion of POSS into polymer matrices can improve both the physical and chemical properties of the material, e.g. an increased glass transition temperature (TG ), an expanded operational temperature range, a higher degradation temperature, oxidation resistance, surface hardening and reduced oxygen permeability. 4,5 These outstanding properties provide opportunities to develop robust materials that can be used in harsh environments. 6 POSS are nano-sized silicon-based nanoparticle resins containing an inner inorganic framework of silicon and oxygen with particle sizes ranging from 1 to 3 nm in diameter. 7,8 The empirical formula of POSS is (RSiO3/2 )n , where n is commonly 4, 6, 8, 10 or 12 for a completely ‘condensed’ silsesquioxane, and R can be hydrogen, inert organic groups or reactive functional groups. 1,3,7,9,10 An example structure of a completely condensed POSS with n = 8 is shown in 1. The substituent 2 ACS Paragon Plus Environment

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‘R’ groups are used to improve solubility in solvents and to enhance miscibility with polymeric host materials. 1,9 Two typical POSS structures when R is a phenyl group (PhPOSS) or a t-butyl group (tBuPOSS) are used in this study. Reactive functional groups permit the POSS cages to be graftable and polymerisable nanoparticles. 4 R

R Si O R

Si O Si

O

O

O Si

Si

O RO

R Si O Si O O O O

R

Si

R

R

Figure 1: Molecular structure of polyhedral oligomeric silsesquioxane (POSS), (RSiO3/2 )n , with n = 8, and R can be hydrogen, inert organic groups or reactive functional groups. POSS particles can be incorporated into polymer matrices in the forms of either physically blended or chemically bonded composites. 2 They have also been used There are several approaches for chemical incorporation of POSS into polymers including monofunctional POSS cages grafted onto polymer chains as a pendant group. 10 Polymerisation techniques such as ring-opening 9,11,12 radical 13,14 or condensation polymerisation 15 may be used to incorporate monofunctional POSS into host polymers. Hydrosilylation 16 is an addition reaction of a silicon hydride moiety across an unsaturated linkage 17 that has been widely used to prepare POSS-based polymer nanocomposites. 1,2,18,19 Similar nanocomposites, prepared from polysilicate-based particles studied previously, 20,21 also have an inner inorganic framework of silicon and oxygen and particle sizes ranging from 1-3 nm. The polysilicates are prepared and used as an amorphous mixture of structures which are grown via an acid catalyzed hydrolysis and condensation process; the size of the structures are regulated through the use of trimethylsiloxy terminating groups and their quantity relative to the silicate structure building units. The trimethylsiloxy groups also provide a methylated particle surface which facilitates dispersibility and compatability with PDMS polymers and matrices. In this study, trimethylsiloxy silicate particles (termed R2) with a molar ratio of trimethylsiloxy groups to 3 ACS Paragon Plus Environment

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silicate groups near 0.95 were used to prepare comparative physical blend nanocomposites. Miscibility between POSS, R2 and PDMS can be predicted by investigating the difference in their solubility parameters, δ , using the Flory interaction parameter theory, with χ :

χ = (ν /RT )(δA − δB )2

(1)

where ν is an arbitrary reference volume (conveniently selected as 100 cm3 ) and δ is the solubility parameter. 22 The solubility parameter of PDMS is δPDMS = 16.5 (J/cm3 )1/2 . 22 For the R2 resin used in this work, the solubility parameter was estimated based on the empirical method developed by Hansen. This consisted of testing the solubility of R2 resin in a range of solvents with known solubility parameters. This results in a solubility parameter for R2 of δMQ = 16.4 (J/cm3 )1/2 which is very close to the value found for PDMS (16.5). Data for PhPOSS δPhPOSS = 19 (J/cm3 )1/2 and tBuPOSS δtBuPOSS = 14 (J/cm3 )1/2 was obtained from Hansen. 23 Comparing solubility parameters shows that R2 and PDMS are expected to be highly miscible while both POSS nano-particles are highly immiscible with PDMS. It is therefore expected that combining R2 or POSS with PDMS will result in very different morphological behaviour. In our group’s earlier work in this area 24–28 we focussed on the interaction of PDMS melts with trimethylsily-treated polysilicate nanoparticles. Both the molecular weight of the polymers used and the size of the nanoparticles were found to be important in determining the local chain mobility (as seen by NMR T2 relaxation and diffusion) and the bulk viscosity of the samples. In particular, small nanoparticles (∼ 0.3 nm) decreased the bulk viscosity (increased the T2 relaxation times) of both high and low molecular weight PDMS. Larger nanoparticles (∼ 2 nm) increased the bulk viscosity of low molecular weight PDMS (reduced T2 ) but below a critical concentration and above a critical molecular weight they decreased the bulk viscosity (increased T2 ). These latter unusual effects can be rationalized in terms of reducing the extent of chain entanglements, 27 polymer adsorption 24 and possible changes in free volume. 29 In particular, 27 the ratio of the the unperturbed radius of gyration (Rg ) of the nanoparticle to the polymer was found to be significant.


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Kopesky et al. 30 investigated nanocomposites consisting of POSS particles tethered as side groups at up to 25 wt% in high molecular weight Polymethylmethacrylate (PMMA) as well as POSS/PMMA nanocomposite blends and compared both to the properties of PMMA alone. Viscosity measurements at temperatures above the TG of PMMA and below the TM of POSS showed that physical blending of POSS with PMMA showed non-Einstein viscosity reduction behavior relative to PMMA alone up to a loading of 5 vol%. Above this loading the POSS particles were found to agglomerate into crystallites that resulted in a viscosity increase of the nanocomposite relative to PMMA alone consistent with behavior predicted for hard spheres. 31 The main interest of this paper is to synthesize and characterize the dynamics of two series of POSS-PDMS-POSS triblock polymers prepared via hydrosilylation reactions and to compare the impact of the POSS particles on the relative mobility of the PDMS block relative to a free PDMS chain. The triblocks are also compared to nanocomposites prepared by simple blending of R2 particles with a low molecular weight PDMS. The size of the triblock polymers was characterized using small-angle neutron scattering (SANS) and pulsed-field gradient stimulated echo (PFG STE) NMR experiments. The molecular mobility of the materials themselves and their incorporation into host polymer melts was also studied by NMR spin-spin (T2 ) relaxation.

Experimental Materials Hydride terminated poly(dimethylsiloxane), (Hydride–PDMS)of 0.58 and 24 kg/mol molar mass, POSS–allyl–heptaisobutyl substituted (Allyl–tBuPOSS) and platinum(0)–1,3–divinyl –1,1,3,3–tetramethyltrisiloxane complex solution in xylene (Karstedt’s catalyst) were purchased from Sigma–Aldrich. Hydride–PDMS of nominal 13.3 kg/mol was supplied by Dow Corning Corp. (USA). Hydride–PDMS 1.17 , 5.54, 16.5 and 41.4 kg/mol (nominal values) were purchased from Gelest Inc. PSS–vinyl–heptaphenyl substituted (Vinyl–PhPOSS) was purchased from Hybrid Plastics. All chemicals were used as received without further purification. 5 ACS Paragon Plus Environment

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Synthesis and characterisation of POSS–PDMS–POSS triblocks: A series of tBuPOSS–PDMS–tBuPOSS (BPB) and PhPOSS–PDMS–PhPOSS (PPP) triblocks were synthesized with different PDMS segment lengths varying from 0.58 to 41.4 kg/mol through hydrosilylation reactions of the corresponding hydride-PDMS and allyl–POSS. Representative synthesis procedures for 1.17 BPB and 41.4 PPP kg/mol triblocks are described. For the other samples suitable adjustments were made for the molecular weight change. Reaction times depended on the molecular weight of the hydride-PDMS. The general scheme of the hydrosilylation reactions is shown in 2. CH3

CH3

Si O

H

Si

O

Si

CH3

O

CH3

Si n

H

R

+

Si O

CH3

O

O

O Si

O

C H

Si

O RO

R Si O Si O O O

Si

CH2

H2 C

R CH3

R

Si

R

R

Karstedt's catalyst/CH2Cl2 100 °C R CH3 R Si O

R Si O Si R

O

O

O Si

Si

O RO

R Si O Si O O O O

H2 C

H2 C

H2 C

Si CH3

CH3 O

Si CH3

R

O

Si n

H2 C

H2 C

O

H2 C

Si O

CH3

Si R

R

Si

O R O O

O Si

Si

O

Si

CH3

Si O

O RO Si

O O

R

Si R

R

Figure 2: General scheme of hydrosilylation reaction to synthesize POSS-PDMS-POSS triblocks

Synthesis of BPB triblocks: 1.17 kg/mol hydride–PDMS (0.683 g, 0.583 mmol) was dissolved in < 20 mL of dichloromethane and introduced to a 100 mL three necked round-bottom flask. The flask was equipped with a magnetic stir bar and a water-cooled condenser. Allyl–tBuPOSS (1.0 g, 1.166 mmol) was then added to the PDMS solution. The reaction mixture was stirred for several minutes to allow the reagents to mix well. 0.022 g Karstedt’s catalyst was dissolved in 2 mL dichloromethane and added. The reaction mixture was refluxed at 100o C under a nitrogen atmosphere. The reaction progress was monitored by FT-IR analysis of samples taken throughout the synthesis using the Si-H band at 2126 cm−1 . After the reaction was complete, the solution was stirred with silica gel and activated carbon for several hours to remove unreacted starting reagents. 32 The mixture was filtered through a glass column containing Celite and Florisil to re6 ACS Paragon Plus Environment

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move Karstedt’s catalyst using 1000 mL of dichloromethane as eluent. The solvent was then removed by rotary evaporation and dried in an oven for several hours at 30o C under vacuum. The product was obtained with a yield of 70%. Other BPB triblocks were obtained with yields of 60 to 80% and were used without further purification. Synthesis of PPP triblocks: A 100 mL three neck round bottom flask was loaded with 41.4 kg/mol hydride–PDMS (21.06 g, 0.508 mmol), vinyl–PhPOSS (1.0 g, 1.017 mmol) and 60 mL toluene. The flask was equipped with a mechanical stirrer, a temperature controller, a water-cooled condenser and a nitrogen atmosphere. The solution was heated to 100o C and stirred for several minutes. 0.0298 g of Karstedt’s catalyst was dissolved in 2 mL toluene then added to the solution. The reaction mixture was refluxed and samples were removed periodically for FT–IR analysis to monitor the reaction progress. After the reaction was complete, the crude product was left to cool to room temperature. The mixture was then filtered through a column of Florisil to remove residual Karstedt’s catalyst using 1000 mL toluene as eluent. Solvent was removed by rotary evaporation followed by vacuum drying at 30o C. The product with a yield of 84% was obtained. Yields of 75 - 90% were obtained for synthesis of other PPP triblocks with different PDMS molecular weights and were used without further purification. Trimethylsiloxy polysilicate nanoparticles: The resin nanoparticles were supplied by Dow Corning Corporation, and are designated here as R2 (representing an intermediate particle size between silicates previously investigated by the authors). 20,33 An acid-catalyzed polymerization of sodium silicate followed by a reaction with trimethylchlorosilane in a process described elsewhere 34 using an equivalent molar ratio of trimethylsiloxy and silicate units produced this amorphous nanoparticle with a weight-average radius of gyration of 1.5 nm and glass transition temperature of 96 ±3 o C by dynamic mechanical thermal analysis.

NMR spectroscopy 1 H NMR measurements of the starting compounds and the reaction products were performed on a Lambda 300 (300 MHz) and a Varian 400 (400 MHz) NMR spectrometers.


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Dichloromethane-d2 was used as solvent. FT-IR spectroscopy FT–IR spectra were measured on a PerkinElmer Spectrum One FT–IR spectrometer. A small amount of the sample was coated on a NaCl plate. Spectra were measured over the wavenumber range from 450 to 5000 cm−1 with an average of 4 scans. Gel permeation chromatography (GPC) The GPC analysis was performed by Dow Corning Corporation. All samples were prepared in HPLC grade toluene at 0.5 % w/v concentration and filtered through a 0.45 µ m PTFE syringe filter. The samples were analyzed against polystyrene standards ranging in molar mass from 0.580 to 1,290 kg/mol. The chromatographic equipment consisted of a Waters 515 pump, a Waters 717 auto sampler and a Waters 2410 differential refractometer. Chromatographic separation was made with two (300 mm × 7.5 mm) Polymer Laboratories PLgel 5 µ m mixed–C columns preceded by a guard column. Analysis was performed using HPLC grade toluene as the eluent with the flow rate of 1.0 mL/min. The columns and detector were both heated to 45o C. Repeatability of the GPC molecular weight measurements was ± 5%. Small-angle neutron scattering (SANS). Samples of hydride-PDMS melts, POSS and POSSPDMS-POSS triblocks were prepared in D-toluene at concentrations of 10 wt% polymer. Smallangle neutron scattering (SANS) measurements were carried out on the D11 instrument at the Institut Laue-Langevin (ILL) in Grenoble, France. All samples were studied at 25o C in 1 mm path length quartz cells and a neutron wavelength of 8 Å was used. The detector distance was 1 m and 2.3 m with neutrons wavelengths between 4 and 10 Å, giving a scattering vector (Q) range of 0.01 Å−1 < Q < 0.4 Å−1 The experimental data were fitted to the Guinier - Debye model for polymers using a non-linear least squares algorithm. Diffusion NMR spectroscopy All samples of the POSS-PDMS-POSS triblocks and their starting materials in D-toluene are the same set as used for SANS measurements. Pulsed–field gradient stimulated echo (PFG STE) NMR experiments were performed on a Bruker DSX-300 NMR spectrometer operating at 300 MHz for protons. The length of the field gradient pulse, δ , was set to 1 ms for all samples. The magnetic field gradient, g, was varied from 0.0496 to 9 T/m. The diffusion time, ∆, was set between 100 and 400 ms. The attenuation of the stimulated spin-echo signals


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was measured by studying the change in the integral of the sample peak area in the spectrum, A(g), as a function of the applied pulsed- field gradient. The self-diffusion coefficients, D, were calculated by using non-linear-least-square fits to the equation for unrestricted diffusion: 35 A(g) = A(g = 0) exp[−γ 2 g2 δ 2 D(∆ − δ /3)]

(2)

where γ is the magnetogyric ratio. Spin-spin relaxation NMR spectroscopy Spin-spin (T2 ) relaxation measurements of all samples of the PPP triblocks and their starting materials were measured using a Bruker MSL 300 MHz spectrometer using the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence 36 with a 180o pulse separation of 2 ms (for PPP triblock polymers) and 4 ms (for hydride-PDMS samples). The signal was averaged over 64 scans with at least 2048 data points sampled at the echo maxima. This reduces errors from random noise and possible DC offsets and makes it possible to fit multiple exponentials to the data. The recycle delay for each scan was set between 10 and 15 s to ensure a full recovery of magnetization between consecutive acquisitions.

Results and discussion The hydrosilylation reaction progress for the two POSS samples and the hydride-PDMS was monitored by FT-IR spectroscopy. The disappearance of the absorption peaks at 2124 to 2126 cm−1 indicated that the Si–H group of the hydride-PDMS had reacted and the reaction was complete. This was after 50 h for tBuPOSS and 4 h for the PhPOSS. The reaction time to completion was found to increase with increasing hydride-PDMS molecular weight. 1 H NMR: 1 H NMR spectra of the hydride-PDMS, allyl-tBuPOSS, vinyl-PhPOSS and the triblocks

were recorded. The completion of the hydrosilylation reaction was confirmed by the absence of peaks from the -CH=CH2 on the allyl-tBuPOSS (δppm 5.75 and 4.90) and the Si-H on the hydridePDMS (δppm 4.69 ) in the final product.

1H

NMR was used to determine the number average

molecular weight (MN ) of the polymers. 37–39 The number of protons in each polymer molecule 9 ACS Paragon Plus Environment

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was determined from the integration of the PDMS peak (δ ppm 0.07) with the integrals calibrated to the end group protons. The calculated MN of all the compounds was obtained from integration of their 1 H NMR spectra and is given in 1. There is good agreement between the MN of hydridePDMS and BPB products suggesting that the desired products were obtained. A similar approach was taken for the PhPOSS samples. Residual vinyl-PhPOSS was not expected because the molar ratio of PDMS to POSS for the synthesis was kept at 1:2 for all the samples, so the starting reagents should have been completely reacted. However, the small signal of –CH=CH2 at δppm 5.96 - 6.28 shows that a small amount of unreacted vinyl–PhPOSS remains in the final product. The quantity of the excess POSS was approximately 2% calculated from the integral value of the peak. Assuming that the small amount of the unreacted vinyl–PhPOSS should not have a significant effect on the properties of the product, no further purification was attempted. A possible explanation for the unexpected excess vinyl–PhPOSS found in the final product is that the reaction mixtures may contain trace amounts of water which could react with hydride–PDMS and form a side product. This side reaction would lead to a decrease in the amount of hydride–PDMS available for the reaction with vinyl-PhPOSS resulting in unreacted POSS. Less than 10 wt% of unreacted vinyl-PhPOSS was observed in all the reaction products. The MN for all the samples was calculated from integration of the 1 H NMR spectra and these data are also shown in 1. GPC The GPC profile of 0.58 kg/mol BPB triblock product in comparison to its starting reagents shows a shift to a higher molecular weight of the reaction product and is consistent with a successful reaction between the starting reagents. A small signal of allyl–tBuPOSS can be seen in the GPC profile of the final product, however, the amount of excess POSS should be very small in comparison to the product as shown by the absence of –CH=CH2 in the 1 H NMR analysis. The GPC curve of 41.4 kg/mol hydride-PDMS shows evidence of a low molecular weight component in the sample. This component could be assigned to a species that is not involved in the hydrosilylation reaction because its molar mass, as shown in 1, does not change significantly at the end of the reaction. MN and polydispersity index (PDI) of some of the starting reagents and BPB triblocks obtained from GPC analysis are shown in 1. The detailed GPC data is given in


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the supplementary information. It can be seen that the hydride-PDMS with MN >10 kg/mol have dispersities > 2, leading to the formation of polydisperse triblock polymers. GPC data for the 13.3 kg/mol PPP triblocks and its starting reagents were recorded; a shift in the GPC trace to a higher molecular weight for the reaction product which is consistent with obtaining the desired product was obtained. A peak at low molecular weight corresponded to vinyl–PhPOSS is observed in the GPC curve of the product. This evidence confirms the excess POSS remaining in the product also seen by 1 H NMR. However, since we are probing the mobility of the protons associated with the linear block only it is believed that small amounts of residual untethered POSS will have a negligible impact and so no further efforts were made to purify the triblocks. In addition, it is felt that excess POSS would have less direct impact on the PDMS proton mobility results than excess linear PDMS which would result in untethered chains. If we assume all the extra particles are distributed uniformly through the sample then a level of 2% extra particles would increase the bulk viscosity by ∼ 5 % and the effect on the NMR relaxation would be similar. The actual level is well below this value. In summary, the PPP and BPB triblock polymers were successfully synthesized through a Pt catalysed hydrosilylation reaction. The likelihood of any appreciable amount of non-functionalized or unfunc-tionalized PDMS remaining in the final product is low based on both the IR and NMR data and the extended reaction times allowed. The final products contained small amounts of unreacted POSS starting materials. Small-angle neutron scattering and pulsed-field gradient NMR Small–angle neutron scattering (SANS) and pulsed–field gradient stimulated–echo (PFG STE) NMR measurements were carried out on the PPP and BPB triblocks and their starting materials. The aim of these experiments was to determine the size of POSS particles, hydride–PDMS and the triblock polymers dispersed in D–toluene. The measurements were performed on 10 wt% solutions. To determine the molecular size of the materials, the data were fitted to the Guinier -


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Table 1: NMR and GPC results of tBuPOSS-PDMS-tBuPOSS-triblocks, PhPOSS-PDMSPhPOSS-triblocks and their starting materials based on polystyrene standards analysed in toluene. Sample Name MN (NMR) g/mol Allyl–tBuPOSS 856 Vinyl–PhPOSS 983 0.58k hydride–PDMS 726 1.17k hydride–PDMS 1170 5.54k hydride–PDMS 5540 13.3k hydride–PDMS 17894 16.5k hydride–PDMS 16488 24.0k hydride–PDMS 25146 41.4k hydride–PDMS 41400 0.58k BPB 2362 1.17k BPB 2880 5.54k BPB 7098 13.3k BPB 13610 16.5k BPB 18494 24.0k BPB 27448 41.4k BPB 33516 0.58k PPP 2764 13.3k PPP 18230 16.5k PPP 20006 24.0k PPP 26296 41.4k PPP 45314 * Not Available

MN g/mol 836 634 820 * * 11600 12500 13800 16600 2320 * * 11900 11600 * 19300 2000 18200 * 16900 18400


MW g/mol 849 646 1090 * * 27900 25800 34900 77400 3410 * * 31100 50800 * 96800 2120 40400 * 90400 133000

PDI 1.02 1.02 1.33 * * 2.41 2.06 2.52 4.66 1.47 * * 2.62 4.37 * 5.01 1.06 2.22 * 5.34 7.23

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Debye model for polymers : I(Q) = (ϕ MW ∆ρN2 /(NA ρ )) exp[−(QRG )2 /3]

(exp[−(QRG )2 ] − 1 + (QRG )2 ) (QRG )4

(3)

where ϕ is the volume fraction of polymer, ∆ρN is the difference in scattering length density of the polymer and solvent, Q is the scattering vector and RG is the polymer radius of gyration, MW the polymer weight average molecular weight and ρ the polymer density. Table 2: Calculated scattering length densities (SLD) of compounds used for SANS measurements. Compound ∆ρN D–toluene Allyl–tBuPOSS Vinyl–PhPOSS Hydride–PDMS

Å−2 /10−6 5.68 0.63 2.19 0.06

Calculated scattering length densities (SLD) of the components used in the SANS experiments are given in 2. In the fitting, values of scattering length density (SLD), volume fraction and density were fixed. The other parameters were allowed to vary. Representative scattering data for 0.58k BPB triblocks and its starting materials are given in 3. The results for the radius of gyration (RG ) for all samples obtained from the Guinier-Debye fits are given in 3. The RG values of hydridePDMS in Å can also be calculated using the relation:

b R2G = aMW

(4)

where MW is weight average molar mass of polymer in g/mol. The constants a = 0.0666 and b = 1.0141 for linear PDMS at 30o C. 40 The Guinier-Debye fit yielded an RG of 0.57 ± 0.01 nm for allyl-tBuPOSS. This value is in agreement with the general size range expected for POSS particles (size ∼ 1 to 3 nm in diameter). 7 Vinyl-PhPOSS was characterized using small-angle X–ray scattering (SAXS) at a concentration of 10 wt% vinyl-PhPOSS in tetrahydrofuran. The RG of the vinyl-PhPOSS was observed as 0.48 ± 0.11 nm which is similar in size to the allyl-tBuPOSS. The RG values obtained for 0.58k 13 ACS Paragon Plus Environment

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10 0.58k hydride-PDMS Allyl-tBuPOSS 0.58k BPB Intensity / cm-1

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1

0.1

0.1 Q / Å-1

Figure 3: Small–angle neutron scattering from BPB triblocks in 10wt% solutions in D-toluene compared to their starting materials. Solid lines are fits to the Guinier-Debye model for polymers. and 5.54k hydride-PDMS are in reasonable agreement with the calculated RG using equation 4 ( 0.88 nm and 2.3 nm) though at higher molecular weights the values are rather smaller than the calculated ones. In Benton’s work, 41 it was found that the RG of linear PDMS decreased with increasing polymer concentration from 0.5 wt% to 10 wt%. The decrease in RG could be attributed to changing from the dilute to semi-dilute solution regime which occurs at c∗ . 41 c∗ ∼ MW /(4π R3G /3)

(5)

For the highest molecular weight sample c∗ is close to 10 vol% and hence could be a reason why the observed sizes in these experiments are smaller than those calculated using equation 4. How0.5 though some anomalous data ever, there is still an approximate linear dependence of RG with MW

were found for the block copolymers.

For the PPP triblocks samples, the PhPOSS particles are attached to both ends of the PDMS


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middle chain and hence the RG values might be expected to be larger than the corresponding hydride-PDMS though not as large as a dumbbell model would predict. This obviously depends on the conformation, but it was found that the RG values for the triblocks polymers were larger than the homopolymers. The difference between the two different particles is small but as the PDMS chain length increases the values approach those of the homopolymers.

Table 3: Radius of gyration from SANS for PDMS and the tri–block copolymers

MN (by NMR) of PDMS / kg/mol 0.726 5.54 17.894 25.146 41.400

PDMS(SANS) /nm 0.79 2.01 2.39 2.46 2.78

BPB /nm 1.25 2.57 2.39 2.77 2.82

PPP /nm 1.03 2.37 2.37 2.80 2.80

Diffusion NMR spectroscopy Pulsed–field gradient stimulated echo (PFG STE) NMR experiments were performed at 25o C for samples of hydride–PDMS and the triblock polymers dispersed in D–toluene and the representative results are shown in 4. The diffusion data were analysed using a non-linear-least-squares fitting routine to equation 2. The hydride–PDMS is a pure polymer, so a single diffusion coefficient was expected to fit to the data (data were fitted to a single gaussian decay though the data for the high molecular weight samples showed deviations likely to be due to polydispersity). It is clear that the gradients of the attenuation data decrease when POSS particles were attached to the PDMS in which is a clear indication of a decrease in the self-diffusion coefficient and, hence, an increase in the hydrodynamic radius. Similar non–linear data were found for the other samples. The hydrodynamic radius, RH , of a compound in a dilute solution can be calculated from the self-diffusion coefficient, Ds , using the Stokes–Einstein relation. 20


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" !

!

!" !

A(g)/A(g=0)

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" ! !

" ! !

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" ! " !

0.1 0.0

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Gradient/ (T/m)

Figure 4: PFG STE NMR data at 25o C of the 0.58 kg/mol samples, º starting polymer , BPB and PPP triblocks. All the samples were prepared at concentration of 10 wt% in D-toluene. The solid lines are fits to a single diffusion coefficient.

Ds =

kB T 6πη RH

(6)

where Ds is the self-diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature (K) and η is the solvent viscosity at the experiment temperature. Plots of RH as a function of the molecular weight of the PDMS segment are shown in 5 and there is a clear increase in RH when the PDMS segment molecular weight increases. Also there is a very clear difference between the two block copolymers, with the PhPOSS based materials having a larger hydrodynamic radius than those based on tBuPOSS. Note that at a solution concentration of 10 wt%, the requirement for dilute solution behavior is likely exceeded. In this case, polymer coil diffusion can be slowed due to hydrodynamic and thermodynamic chain interactions. In the case of the PPP triiblocks, PI-PI stacking could be expected to result in extended conformations leading to the higher RH values. The SANS data also show a small difference in the two block copolymers. RG and RH for a linear polymer in dilute solution are expected to follow a simple relationship but neither the neutron or NMR experiments are in the dilute solution regime. 42,43 Comparison of the data in 5 and 3 show that the RH values are much larger than the RG values,


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Figure 5: Hydrodynamic radius as a function of molar mass of the PDMS segment weight: (º) homopolymer PDMS, () BPB and () PPP triblocks. The lines are to guide the eye. All the samples were prepared at concentration of 10 wt% in D-toluene . especially for the triblock copolymers. There are several potential reasons for this including both intermolecular and hydrodynamic effects; the sample concentrations are approaching c∗ (the dilute concentration limit) leading to intermolecular interactions between POSS moieties. Note that the solvent, toluene, will ensure that both blocks are molecularly dispersed which was confirmed by the optically clear appearance of the solutions. In contrast, the next section will explore the other extreme of polymer melt behavior where intermolecular interactions will be maximized and solubility differences between the POSS or R2 and PDMS constituents will dictate the morphology and with it the block copolymer or nano-composite relaxation behavior. T2 relaxation NMR spectroscopy of polymer melts and mixtures.

The T2 relaxation of the

BPB and PPP triblocks and their corresponding starting materials in the melt state were measured to study their molecular mobility. The relaxation decay curves are shown in 6 and 7. The decays were not single exponential as found before, but could be fitted to multiple exponentials. 26 There are several potential reasons why non-exponential behaviour is seen in these systems and they are related in part to the different physical environments the polymer segments find themselves in. For example, those segments close to the nanoparticle surface will have their mobility reduced com-


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pared to those away from the interface. 44 Note, the protons in the nanoparticles in these melt samples will not make a substantial contribution to the relaxation decays as the CPMG method does not refocus strongly dipolar coupled protons. 44 A single exponential relaxation would only occur in a single phase spin system in which the protons could reorientate isotropically and for polymeric systems that is unlikely. Several publications cite non-exponential behaviour for polymer systems though the techniques and samples available for this work preclude a more detailed study. The samples are also quite polydisperse (Table 1) and this will also influence the absolute values of the relaxation time components. However, as we have shown in previous publications 20,21 a good qualitative description can be made by using one of (a) an averaged relaxation rate (b) the reciprocal time value at 1/e of the decay or (c) just simply the fastest relaxing component. This last value is plotted in the subsequent figures.

Figure 6: Spin-spin relaxation decays for the series of PPP triblock copolymers A qualitative look at these two figures shows that the PPP samples have much faster decays especially for the lowest molecular weight samples which have the highest nanoparticle concentration. 8 shows plots of the spin-spin relaxation rate coefficient (1/T2 ) as a function of molar mass 18 ACS Paragon Plus Environment

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Figure 7: Spin-spin relaxation decays for the series of BPB triblock copolymers of the PDMS segment: note reduced molecular mobility is indicated by an increase in relaxation rate. The relaxation rate coefficient of hydride-PDMS increases with increasing molecular weight as expected. 21,45 Considering a series of the triblock copolymers containing the same length of PDMS central segment, the relaxation rates of the triblocks are higher than that of hydride-PDMS in particular for the case of triblocks with short PDMS chains. This suggests that the two POSS particles attached to the central chain act as anchors and suppress the molecular motions of the chains. Overall, the BPB triblocks are more mobile than PPP triblocks and this is due to the effect of the rigid benzene rings in the PhPOSS end groups. The aromatic rings from different PhPOSS particles may also form clusters because of their incompatibility with PDMS which could restrict the motions of the PDMS middle blocks. In contrast to the hydride-PDMS, the relaxation rates of BPB and PPP triblocks decrease with increasing molecular weight of the PDMS segment. This means that the triblock copolymers are more mobile as the central block becomes longer and dominates. However, the molecular weight effect seen for the homopolymer can still make a contribution. At high chain lengths these materials all converge towards a constant 1/T2 value. The relaxation rate of PPP triblocks as a function of weight fraction of the POSS component


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%&!"'()"*+,-! Figure 8: Relaxation rate coefficients as a function of molar mass of the PDMS segment. Lines are not fits to the data; they are guides to the eye. (º) homopolymer PDMS, () BPB, and () PPP triblock copolymers. with respect to the pure polymers is shown in 9. Clearly, the relaxation rate increases with increasing the effective POSS content, indicating that the triblock copolymers are less mobile even though the polymer molecular weight is decreasing. We also show data for a nano-sized trimethylsilylfunctionalized polysilicate (R2) resin dispersed at different concentrations in a 10 cSt PDMS (approx molar mass 1.3 kg/mol). The R2 resin particles used in this work were trimethylsilylfunctionalized to avoid the complication of interfacial interactions and agglomeration of the PDMS melt and the particles. At low resin contents (high molecular weight PDMS for PPP and BPB) the block copolymers are considerably less mobile than the free resin samples but the values converge as the percentage of resin content increases. Note that the difference in the data at low nanoparticle concentration is due to the difference in molecular weight of the triblock and hompolymer samples and this effect will become less significant as the nanoparticle concentration increases. The difference in compatibility between the nanoparticles is also important as it is likely the triblock copolymers may aggregate forming large clusters, whereas the R2 resin should be better dispersed. Other studies on POSS end-capped poly(propylene oxide) suggest a degree of crystallisation but given the optical transparency of our samples and the dramatic effect of the POSS on 20 ACS Paragon Plus Environment

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&!'()*+(,-./0! Figure 9: The specific relaxation rate as a function of wt% of the POSS nanoparticle component. Lines are not fits to the data; they are guides to the PPP, BPB, ♢ are the resin R2 in PDMS dispersions . the local segmental dynamics leads us to conclude that this does not occur to any appreciable extent in the present system. The R2 system shows a strong increase in relaxation rate above 60 wt% resin. SANS evaluation of the trimethylsiloxy polysilicate nanoparticles previously 27 revealed that molecules in the MW range of R2 (MW = 5.6 kg/mol) have spherical geometries trending toward short cylindrical structures with an increase in MW . The rapid increase in spin-spin relaxation rates of the polymer protons near 65 wt% (61 vol%) R2 nanoparticle loading is hypothesized to be due to a percolation threshold above which the mobility of the polymer chains are severely restricted by the close packing of the R2 nanoparticles. Powell, 46 through lattice percolation investigations using methods similar to those developed previously 47 predicted a percolation threshold for a step change in conductivity from conductive spherical particles in a polymer network to be near 31 vol%. On the other hand, the close packing limit for spherical particles in a cube has been shown to be near 74 vol%. 48 Hence, it is not unreasonable to expect that a percolation threshold for a step change in reduced polymer mobility would fall between these benchmarks for percolation/packing behavior. 21 ACS Paragon Plus Environment

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Conclusions The triblock polymers with two different pendant groups on the POSS particles, i.e. isobutyl and phenyl groups, were successfully synthesized through the hydrosilylation reaction using Karstedt’s catalyst. The conformal behavior of the linear PDMS and their corresponding triblock polymers was determined in dilute solution by small-angle neutron scattering (SANS) and by self diffusion NMR. The radius of gyration (RG ) from SANS and the hydrodynamic radius (RH ) from NMR exhibit a clear increase with PDMS segment length, both for the homo polymer and triblock copolymers. The PPP triblocks have a larger RH compared to the BPB triblocks and the homopolymer. This could be due to the expected higher hydrodynamic and thermodynamic chain interactions for PPP. The RG of the triblock polymers was dominated by the PDMS middle chain when the length of the middle chain increased. The mobility of the triblock copolymers PPP and BPB, as determined in the melt, is much lower than the PDMS homopolymer and PDMS melts reinforced with non-grafted compatible polysilicate nanoparticles (R2). Therefore, grafted units that have a tendency to form a separate rigid phase are more effective at reinforcing PDMS than occurs when blending a compatible rigid nanoparticle like R2. In line with the results in dilute solutions, the PhPOSS blocks in PPP are more effective at reinforcing or reducing the mobility of PDMS than the tBuPOSS blocks in BPB. For the blended R2/PDMS nanocomposites, a rapid increase in spin-spin relaxation rates was seen for PDMS protons near 65 wt% (61 vol%) R2 nanoparticle loading and can be hypothesized to be due to a percolation threshold above which the mobility of the polymer chains are severely restricted by the close packing of the R2 nanoparticles. The new copolymers are miscible with PDMS itself and they can act as an easily dispersable reinforcing and film forming agent, without the problems of particle aggregation which are common in non-aqueous systems. The new copolymers can also find use as protective encapsulants for opto-electronic devices. 49


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Acknowledgement The authors would like to thank, Dr. Beatrice Cattoz and Dr. Shirin Alexander, School of Chemistry, University of Bristol, Bristol, U.K., for performing SANS measurements and the Institut Laue-Langevin (ILL), Grenoble, France for SANS beam time and facilities. We also would like to acknowledge Prof. Robert Richardson, School of Physics, University of Bristol, Bristol, U.K., for carrying out the SAXS analysis used for characterisation of vinyl-PhPOSS size.

Supporting Information Available This information is available free of charge via the Internet at http://pubs.acs.org/. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Pan, G.; Mark, J. E.; Schaefer, D. W. Synthesis and characterization of fillers of controlled structure based on polyhedral oligomeric silsesquioxane cages and their use in reinforcing siloxane elastomers. Journal of Polymer Science Part B: Polymer Physics 2003, 41, 3314– 3323. (2) Baumann, T. F.; Jones, T. V.; Wilson, T.; Saab, A. P.; Maxwell, R. S. Synthesis and characterization of novel PDMS nanocomposites using POSS derivatives as cross-linking filler. Journal of Polymer Science Part A: Polymer Chemistry 2009, 47, 2589–2596. (3) Gnanasekaran, D.; Madhavan, K.; Reddy, B. S. R. Developments of polyhedral oligomeric silsesquioxanes (POSS), POSS nanocomposites and their applications: a review. Journal of Scientific and Industrial Research 2009, 68, 437–464. (4) Pielichowski, K.; Njuguna, J.; Janowski, B.; Pielichowski, J. Polyhedral oligomeric silsesquioxanes (POSS)-containing nanohybrid polymers. Advances In Polymer Sciences 2006, 225–296. 23 ACS Paragon Plus Environment

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Norris, A.;

Swier, S. Organosiloxane block copolymer. 2012;

http://www.google.com/patents/WO2012040367A1?cl=un, App. PCT/US2011/052,615. 28 ACS Paragon Plus Environment

WO

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TOC Entry

Si C O H

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Pt-catalysed hydrosilylation

Hydride-PDMS Phenyl POSS

PPP


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Graphical TOC Entry Some journals require a graphical entry for the Table of Contents. This should be laid out “print ready” so that the sizing of the text is correct. Inside the tocentry environment, the font used is Helvetica 8 pt, as required by Journal of the American Chemical Society. The surrounding frame is 9 cm by 3.5 cm, which is the maximum permitted for Journal of the American Chemical Society graphical table of content entries. The box will not resize if the content is too big: instead it will overflow the edge of the box. This box and the associated title will always be printed on a separate page at the end of the document.


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