Facile Synthesis of Thermoresponsive Poly(NIPAAm-g-PDMS

5 hours ago - Department of Chemistry and Biochemistry, Central Michigan University, Dow Science Complex 263, Mount Pleasant , Michigan 48859 , United...
0 downloads 0 Views 4MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Facile Synthesis of Thermoresponsive Poly(NIPAAm‑g‑PDMS) Copolymers Using Room Temperature Alkylborane Chemistry Robert O. Huber,†,‡ Jeremy M. Beebe,† Patrick B. Smith,§ Bob A. Howell,‡ and Dongchan Ahn*,†,‡ †

The Dow Chemical Company (formerly Dow Corning Corporation), 2200 West Salzburg Road, Midland, Michigan 48686, United States ‡ Department of Chemistry and Biochemistry, Central Michigan University, Dow Science Complex 263, Mount Pleasant, Michigan 48859, United States § Michigan State University, 1910 West St. Andrews Road, Midland, Michigan 48640, United States S Supporting Information *

ABSTRACT: Synthesis of poly(NIPAAm-g-PDMS) polymers using monomethacrylate-functional PDMS macromonomers was carried out over a range of compositions and graft lengths using a facile ambient initiation scheme based on the decomplexation of stabilized alkylborane−amine complexes that is triggered by simple mixing. The copolymerization of NIPAAm with MA-PDMS by this technique was confirmed by monitoring the reaction kinetics and estimating the reactivity ratios via online 1H NMR spectroscopy. These copolymers were confirmed to contain immiscible grafts that are dispersed into nanosegregated domains by AFM, with supporting DSC analysis showing separate Tgs for each polymer phase. DSC studies also show that both the monomer ratio and the PDMS graft length affect the locus of composition-dependent demixing temperature (Tdem) points that delineates the LCST behavior of the copolymer in water. The contrasting impact of low molecular weight (Mn ∼ 1000 Da) PDMS grafts and higher molecular weight (∼5000 and ∼120 000 Da) PDMS grafts on copolymer Tdem are consistent with the frequency of incorporation of hydrophobic graft segments into the acrylic main chain. These results demonstrate the feasibility of using room temperature alkylborane chemistry as a simple, convenient route for producing a variety of thermoresponsive siloxane− acrylamide copolymers that span a broad range of physical and hydrothermal properties.

■

INTRODUCTION

It is of practical interest to covalently integrate such thermoresponsive polymers with organopolysiloxanes such as poly(dimethylsiloxane) (PDMS) because of their ability to introduce thermal stability, flexibility, and low surface tension, along with unique transport properties such as high gas permeability and low viscosity. Although the incorporation of siloxanes into pNIPAAm systems has been variously reported using alkoxysilane precursors,7,8 these systems actually yield rigid silicate domains when cross-linked as opposed to flexible linear siloxane domains. The incorporation of PDMS into pNIPAAm systems to form an interpenetrating network (IPN) or “semi-IPNs” has been reported;9,10 however, this does not involve direct coupling of PDMS to the pNIPAAm main chain. The incorporation of linear PDMS through copolymerization with NIPAAm has also been reported11,12 but at less than 10 wt % PDMS. Recently, thermoresponsive triblock copolymers with a central PDMS block (ca. 28 wt %) have also been reported.13 The work described herein has demonstrated the copolymerization of linear PDMS segments with NIPAAm over a large compositional range (to over 85 wt % PDMS

Thermoresponsive polymer systems have been investigated for a variety of applications including controlled drug delivery,1 biomaterials,2 and membrane separations.3 Aqueous solutions of thermoresponsive polymers exhibit a coil-to-globule transition when heated to their demixing temperature (Tdem),4 resulting in large reversible conformational changes with small changes in the temperature of the system.5,6 Several polymers have been investigated for thermoresponsive characteristics including N-alkyl acrylamides, poly(N-vinylcaprolactam), poly(propylene oxide), poly(vinyl methyl ether), celluloses, poly(2-dialkylamino)ethyl methacrylates), and poly(2-alkyl-2-oxazolines).5 However, the most widely investigated is poly(N-isopropylacrylamide) (pNIPAAm). PNIPAAm derives its thermoresponsive nature from an aqueous lower critical solution temperature (LCST) near 32 °C,5−7 making it conveniently accessible for thermal activation by human metabolic processes. The singular LCST, defined as the minimum of the curve of Tdem vs polymer composition, and the locus of composition-specific Tdem points, are dependent on both the polymer system and the solvent and can be tuned through copolymerization. The incorporation of hydrophilic monomers typically is found to increase Tdem.5 © XXXX American Chemical Society

Received: February 2, 2018 Revised: May 15, 2018

A

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules incorporation) for the first time using a facile ambient alkylborane−initiator polymerization scheme that has been referred to previously as room temperature alkylborane (RTA) polymerization.14 Organoborane compounds are known to be effective polymerization initiators;15−17 however, the most reactive, common lower trialkylboranes are pyrophoric, as they undergo autoxidation at room temperature resulting in the formation of radicals.17 For that reason, unmodified alkylboranes are not widely used for polymerization. To stabilize the initiator, a Lewis base, such as a primary amine, can be used to form a complex with the organoborane.18 Following a previously developed approach,19 National Starch, 3M, Dow Chemical, and Dow Corning have separately developed polymerizable formulations primarily for rigid structural adhesives with a variety of amine-stabilized organoborane complexes, which can subsequently be decomplexed by reaction with an aminereactive compound liberating the organoborane.18,20−22 The consumption of oxygen by the initiator has been applied to reduce inhibition effects in the polymerization of films such as coatings or membranes.23 Additional features and the process flexibility afforded by this chemistry have been highlighted for applications such as copolymerization of pressure-sensitive adhesives, hydrophilically modified microparticles, and threedimensional digital printing and patterning.14 One of the distinct advantages of using amine-stabilized organoborane complexes over other techniques is that they can be decomplexed rapidly by a variety of common amine-reactive compounds under ambient conditions, eliminating the need for heat or radiation. This mixing-induced initiation scheme offers a facile route to produce a variety of polymers with several unique benefits. However, because the bulk of the prior research has focused on cross-linked networks, relatively little characterization of linear copolymers produced by the RTA technique has been reported. In this report the features of the homopolymerization of NIPAAm and its copolymerization with methacrylate-terminated PDMS are described. The feasibility of producing a variety of useful linear PDMS−PNIPAAm copolymers by this facile method has been demonstrated.

■

dried on a Teflon release liner overnight and then stored in a desiccator prior to preparing the NMR samples in CDCl3. Samples that were still hazy were found to not be soluble in chloroform; however, when completely dry and present as a clear film, the material became soluble. Synthesis of Poly(NIPAAm-g-PDMS). To a 40 mL scintillation vial, 2.916 g (25.77 mmol) of NIPAAm, 0.684 g (0.684 mmol) of MAPDMS-1K (MW = 1000 g/mol), 5.844 g of THF, and 0.571 g (2.63 mmol of boron equiv, vinyl:boron ratio 10.0) of TEB-MOPA were added along with a magnetic stir bar. While mixing at room temperature, a solution of 0.420 g (6.990 mmol) of HOAc in 9.574 g of THF was added. This material targeted a [NIPAAm]/[PDMS] ratio of 40. Additional materials with MA-PDMS-1K targeting [NIPAAm]/[PDMS] ratio of 20 and 10; MA-PDMS-5K targeting [NIPAAm]/[PDMS] of 80, 40, 20, and 10; and MA-PDMS-12K targeting [NIPAAm]/[PDMS] of 160, 80, 40, and 20 were prepared. All materials were prepared targeting the vinyl:boron ratio of approximately 10. Characterization Methods. 1H NMR Spectroscopy. Samples were prepared by dissolving approximately 50 mg of dry polymer in approximately 2 g of CDCl3 and analyzed using an Agilent 400 MHz NMR spectrometer at 25 °C. Acquisition time = 5 s, relaxation time = 5−15 s, and nt = 16−64. For conversion samples, approximately 250 mg of polymer solution was added to a 4 mL glass vial, and the solvent was reduced under a stream of nitrogen prior to adding approximately 2 g of CDCl3. Analysis of NMR spectra was performed in ACD Spectrus Processor 2015. 29 Si NMR Spectroscopy. Samples were prepared by dissolving approximately 2 g of dry polymer in approximately 5.5 g of CDCl3 (0.93% w/w Cr(acac)3) and analyzed using 15 mm silicon-free Wilmad PTFE tubes using a Mercury VX 400 MHz FT-NMR spectrometer with a frequency of 79.49 MHz. Acquisition time = 1.5996 s, relaxation time = 13 s, and nt = 240. Analysis of NMR spectra was performed in ACD Spectrus Processor 2015. Annotated spectra showing details of the 1H and 29Si NMR analysis and quantitation are provided in the Supporting Information, section S4 (Figures S4.1−S4.5). Gel Permeation Chromatography. Copolymer samples were prepared at approximately 3 mg/mL and filtered through a 0.45 μm PTFE filter prior to analysis using a Viscotek GPC Max and triple detection (differential refractometer, online differential pressure viscometer, and a light scattering detector). The method utilized two Agilent PL Gel Mixed C 7.5 × 300 mm, 5 μm columns and a PL Gel guard 7.5 × 50 mm column at 35 °C with a mobile phase of THF containing 5% triethylamine and a flow rate of 1.0 mL/min. The sample injection volume was 100 μL, and the run time was 30 min. The MA-PDMS macromonomer samples were characterized using toluene as the eluent. Details of the GPC analysis are provided in the Supporting Information, section S3 (Tables S3.1−S3.5 and Figures S3.1−S3.2). Differential Scanning Calorimetry. Glass transition temperatures (Tgs) as well as crystallization and melt temperatures of dry homopolymers and copolymers were determined using differential scanning calorimetry (DSC) with a TA Q2000 DSC instrument equipped with a liquid nitrogen cooling system. For this analysis, samples (5−10 mg) were weighed into aluminum Tzero Hermetic pans and sealed, and the pan was pierced. The samples were equilibrated at 25 °C under a constant helium purge (25 mL/min) and then heated to 170 °C at 10 °C/min and held for 5 min to erase any thermal history. The materials were then cooled to −150 °C at 10 °C/ min and held for 2 min to equilibrate the baseline and then heated to 170 °C at 10 °C/min to measure the Tgs of the polymers. The demixing temperature (Tdem) of the hydrated homopolymers and copolymers was measured using a second DSC experiment. For this test, samples (3−5 mg along with 10−20 mg of DI water for the copolymers or 6−12 mg of 5% polymer solution in DI water for the pNIPAAm homopolymers) were weighed into aluminum Tzero hermetic pans and sealed. The samples were equilibrated at 25 °C under a constant nitrogen purge (50 mL/min) and then cooled to 10 °C at 10 °C/min and held for 2 min. The material was then heated to

EXPERIMENTAL SECTION

Materials. N-Isopropylacrylamide (NIPAAm, >99%) was received from Sigma-Aldrich (St. Louis, MO). Monomethacryloxypropylterminated polydimethylsiloxane macromonomers (MA-PDMS-1K, MA-PDMS-5K, and MA-PDMS-12K) were received from Gelest (Morrisville, PA). The triethylborane−methoxypropylamine complex (TEB-MOPA) was received from Akzo-Nobel (Chicago, IL). Ethyl acetate (HPLC grade; EtOAc), tetrahydrofuran (HPLC grade; THF), and glacial acetic acid (HOAc) were received from Fisher Scientific (Pittsburgh, PA). All materials were used as received without further purification. Methods. Synthesis of Poly(NIPAAm). To a 40 mL scintillation vial, 1.80 g (15.96 mmol) of NIPAAm, 3.13 g of EtOAc, and 0.119 g (0.55 mmol of boron equiv, [M]/[I]0.5 = 7.18) of TEB-MOPA were added along with a magnetic stir bar. While mixing at room temperature under a dry N2 sweep, a solution of 0.085 g (1.42 mmol) of HOAc in 4.92 g of EtOAc was added dropwise over a period of 2−3 min. The solution was allowed to mix for approximately 40 min until the viscosity increase caused the stir bar to stop spinning. Additional materials with varying initiator concentrations to achieve [M]/[I]0.5 of 5.05, 4.10, and 3.53 were prepared. Samples of pNIPAAm were precipitated into diethyl ether to remove the residual monomer and subsequently dissolved in ethyl acetate and precipitated in deionized water to remove residual initiator and decomplexer. Both precipitations were conducted at room temperature. Samples were B

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules 60 °C at 10 °C/min and held for 2 min prior to cooling to 10 °C at 10 °C/min. This thermal loop was completed twice, and the peak of the measured endotherm of the second cycle was reported as the Tdem. Analysis of the results was performed using TA Universal Analysis 2000 Version 4.5A software. Thermograms showing details of the analysis are provided in the Supporting Information, sections S1 and S2 (Figures S1.1−S1.7 and S2.1−S2.4). Atomic Force Microscopy. Copolymers were dissolved in propylene glycol monomethyl ether at 2.5 wt %. Polymer solutions were filtered through a 0.2 μm PTFE syringe filter onto dust-free silicon wafers and spin-coated at 2000−3000 rpm for 40 s. Samples were analyzed using a Bruker Dimension 5000 atomic force microscope (AFM), operated in TappingMode. Height and phase contrast images were collected at a scan rate of 2 Hz using Bruker TESP cantilevers (nominal spring constant of 42 N/m). For all samples, one 2 μm image was acquired for direct morphology comparison with other samples. An additional image at a different scan size was often collected to either better measure domain sizes or to observe morphology over a broader area. Reactivity Ratio Determination. The reactivity ratios for the NIPAAm and the PDMS macromonomer were determined by conducting the polymerization in a 5 mm NMR tube with a coaxial configuration and monitoring the reaction using 1H NMR spectroscopy. The reaction mixture was loaded into separate sides of a Sulzer Mixpac 5 mL K system micromixing syringe cartridge fitted with a static mixing tip (DN 2.5 × 12, 4:1/10:1) and injected into a 5 mm NMR tube. A 3 mm NMR tube filled with acetone-d6 was inserted to provide a lock for the instrument. The monomer conversion was monitored using an Agilent 400 MHz NMR spectrometer at 25 °C with a frequency of 399.8 MHz. Acquisition time = 2.5−5 s, relaxation time = 5−15 s, and nt = 8−16. To gain better sensitivity for the vinylic proton signals, a WET pulse sequence was used to mask the ethyl acetate signals. Further details on the kinetic analysis for reactivity ratio determination are provided in the Supporting Information, section S5 (Figures S5.1−S5.3 and Tables S5.2−S5.2).

Table 1. Feed Compositions for Poly(NIPAAm) Prepared with Varying TEB-MOPA Concentrations and Molecular Weight Analysis Obtained by Triple Detection GPC and Glass Transition and Tdem Obtained by DSC boron (ppm)

[M] (mol/L)

[I] (mol/L)

Mn (kDa)

ĐM

Tg (°C)

Tdem (°C)

H1 H2 H3 H4

586 1003 1462 1951

1.50 1.48 1.47 1.45

0.04 0.09 0.13 0.17

508 332 221 173

1.74 2.12 2.19 2.21

a 140.4 142.6 142.6

a 32.4 32.4 32.4

a

Samples not tested.

Figure 1. pNIPAAm Mn with respect to [M]/[I]0.5 polymerized at room temperature in ethyl acetate for the organoborane initiated polymers. The dotted line represents a linear regression fit (R2 = 0.9983).

pNIPAAm homopolymers synthesized using the organoborane initiation system, indicating that polymerization utilizing the organoborane initiator follows the same general trend observed for conventional radical initiators. These trends with the acrylamide monomer hompolymerization are qualitatively consistent with reports of similar, less precise trends, observed in an earlier study with an acrylate and methacrylate monomer mixture performed without environmental control.14 The dry polymer was analyzed using DSC to determine the Tg. Subsequently, 5 wt % solutions were prepared in DI water and analyzed by DSC to determine the Tdem. Structural changes during the coil-to-globule transition of the polymer result in an endotherm that can be observed via DSC due to the heat of desorption of the water from the polymer as the sample is heated through the Tdem (see Supporting Information Figures S2.1−S2.4). The H2−H4 polymers all showed similar Tg values between 140 and 143 °C, and the Tdem’s were all similar at 32.4 °C as indicated by the peak temperature of the endotherm. The observed Tdem is very close to the previously reported LCST (32 °C).5 These data show that the MW and dispersity of the polymers do not have a significant impact on the Tg or Tdem, in the MW ranges evaluated. These materials were analyzed using 1H NMR to obtain peak assignments for the pNIPAAm homopolymer (Figure 2) for future determination of conversions and compositions of pNIPAAm−PDMS copolymers. Characteristic peaks for this polymer include the following: δ 1.12 (−CH(CH3)2), δ 1.61 and 1.78 (−CH 2 −), δ 2.13 (−CH−), and δ 3.97 (−CH(CH3)2). It should be noted that after precipitation peaks at δ 5.60 and 6.26 (CH2=CH−) and δ 6.16 (CH2 CH−) are not observed, indicating that all of the residual NIPAAm monomer has been removed. Monomer Reactivity Ratios. The reactivity ratios for NIPAAm and MA-PDMS were determined using online 1H NMR spectroscopy to monitor the reaction as previously described.25−27 Spectra were recorded as a function of time,

■

RESULTS AND DISCUSSION Poly(NIPAAm) Homopolymerization. In radical polymerization, the polymer molecular weight, as represented by its number-average molecular weight (Mn), achieved is related to the concentration of the monomer and the initiator in solution. The generally accepted relationship, under a steady-state assumption with minimal chain transfer reactions, is that the Mn is proportional to the monomer concentration and inversely proportional to the square root of the initiator concentration as shown in eq 1.24 M n ∝ [M]/[I]0.5

sample

(1)

A series of pNIPAAm materials were prepared in ethyl acetate in the presence of various concentrations of organoborane initiator, shown as ppm boron (w/w relative to total solution weight) for comparison across the various substituted alkylboranes. The mole ratio of decomplexer (acetic acid) to the amine (methoxypropylamine) in the TEB-MOPA complex was kept constant at 2. After isolation, samples were analyzed using triple detection GPC to determine the various molecular weight averages and dispersity (ĐM) and DSC to determine the Tg and Tdem (5 wt % solutions in DI water) of the polymers. A summary of the results of these analyses is provided in Table 1. It is interesting to note the high molecular weight (508 kDa) achieved at the lowest concentration of boron while maintaining a relatively low ĐM (1.74) with such a crude synthesis. Aside from a dry nitrogen sweep of the headspace, no special measures were taken, such as rigorous purification of reagents, to achieve these results. A plot of Mn with respect to [M]/[I]0.5 (Figure 1) shows a linear relationship for the C

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

excess. The calculated reactivity ratios for this polymerization system are provided in Table 2 and are comparable to values reported for similar systems.25,26 ⎛ [NIPAAm]0 ⎞ rNIPAAm = ln⎜ ⎟ ⎝ [NIPAAm] ⎠

⎛ [MA‐PDMS]0 ⎞ ln⎜ ⎟ ⎝ [MA‐PDMS] ⎠

⎛ [MA‐PDMS]0 ⎞ rMA‐PDMS = ln⎜ ⎟ ⎝ [MA‐PDMS] ⎠

(2)

⎛ [NIPAAm]0 ⎞ ln⎜ ⎟ ⎝ [NIPAAm] ⎠

(3)

Table 2. Reactivity Ratios for NIPAAm with Methacrylate Functional Comonomers

Figure 2. 1H NMR spectrum in CDCl3 of pNIPAAm synthesized using 2000 ppm TEB-MOPA; chloroform was used as a reference.

reactivity ratio

and peak averaging of the NIPAAm proton peaks at δ 5.66 and 4.97, and the MA-PDMS geminal protons at δ 5.54 and 5.03 were used to track the monomer conversions using the acetoned6 peak (δ 2.05) or the ethyl acetate satellite peaks (δ 4.55) as an internal reference for integration. The monomer reactivity ratios were then determined using these conversion data by applying the Jaacks method28 as previously reported29,30 using eqs 2 and 3. Figure 3 shows the

comonomer diethylene glycol monomethyl ether monomethacrylate glycidyl methacrylate (MA-PDMS-1K)

rNIPAAm

rcomonomer

ref

0.70

2.00

23

0.30 0.91 ± 0.03

2.66 1.83 ± 0.15

24 this work

These reactivity ratios indicate that the MA-PDMS macromonomer will copolymerize with NIPAAm in a somewhat more random manner than either the organic methacrylate monomer systems reported previously. However, since the MAPDMS is more self-reactive, this monomer will be preferentially added, and some compositional drift will still occur as the reaction proceeds in a batch polymerization. Synthesis of Graft Copolymers. A grafting-through technique to synthesize poly(NIPAAm-g-PDMS) was conducted using PDMS macromonomers and a trialkyl organoborane initiator as shown in Scheme 1. Determination of the Mn of the MA-PDMS macromonomers was needed to calculate the molar ratio of the feed compositions. These values were confirmed using 29Si NMR, 1 H NMR, and triple detection GPC analyses. For 1H NMR (Figure 5A), the Mn of the macromonomer was calculated by using the peak integrals for the methyl (δ 0.89, −CH2CH3) and methylene (δ 1.32, −CH2CH2CH3) proton absorptions of the butyl end-group and the methyl (δ 0.07, −Si(CH3)2O−) proton absorptions of the siloxane units. Quantitative termination with the methacryloxypropyl end-group was determined by comparing the peak integrations of the absorption associated with the methylene protons of the butyl end-group with that of the geminal methacrylate protons (δ 6.11 and δ 5.55, CH2C). Likewise, 29Si NMR (Figure 5B) was used to calculate the Mn of the macromonomer by using the integrals for the three silicon peaks: Mbutyl (δ 7.6), MMA (δ 7.4), and D (δ −21.9). A summary of the Mn obtained from these analyses is shown in Table 3. The data from 1H NMR, 29 Si NMR, and GPC provided similar results for Mn. The molecular weight results calculated from the 1H NMR data were utilized for calculating feed ratios to be consistent with the technique used for calculating the final polymer compositions. The radical copolymerization of NIPAAm and MA-PDMS was carried out in THF at room temperature. The reactions were initiated using TEB-MOPA and acetic acid as the decomplexing agent. At the end of the reaction, the polymers were precipitated in deionized water to remove the residual initiator, decomplexer, and unchanged NIPAAm. The materials were isolated and dried overnight at 115 °C and reduced pressure. The copolymer compositions were confirmed by 1H

Figure 3. Time−conversion plot of the TEB-MOPA initiated copolymerization of NIPAAm (●) and MA-PDMS (■, MA-PDMS1K); [NIPAAm]/[MA-PDMS] = 0.11 in ethyl acetate.

time−conversion plots for copolymer generation with [NIPAAm]/[MA-PDMS] mole ratio of 0.11. From this plot, it can be seen that the conversion of the MA-PDMS occurs at a faster rate than the NIPAAm. It can also be observed in these plots that there is a downward curvature with increasing time, indicating a decrease in [M*] due to termination. Jaacks plots were created using these conversion data and are shown in Figure 4. The Jaacks plots show the relative conversion of the excess monomer with respect to the comonomer and the slope of the line is taken as the reactivity ratio of the monomer in

Figure 4. Jaacks plot obtained from the time conversion data in Figure 3 used to calculate the reactivity ratio for MA-PDMS-1K (rMA‑PDMS) when copolymerized with NIPAAm. D

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Synthesis of Poly(NIPAAm-g-PDMS) Using Organoborane Initiation, Top Portion Depicting the Decomplexation of the TEB-MOPA and Subsequent Autoxidation Resulting in Radical Formation

Figure 5. Monomethacryloxypropyl-terminated polydimethylsiloxane (MA-PDMS-1K): (A) 1H NMR; (B) 29Si NMR.

Table 3. Number-Average Molecular Weights Obtained from 1H NMR, 29Si NMR, and GPC for the MA-PDMS Macromonomer Series H NMRb

Si NMRc

1 a

material

MW

MA-PDMS-1K MA-PDMS-5K MA-PDMS-12K

800−1000 5000 10000

29

GPC

Mbutyl

MMA

D

Mn

Mbutyl

MMA

D

Mn

Mn

ĐM

1.0 1.0 1.2

1.0 1.0 1.0

9.3 57.1 156.7

1003 4543 11936

1.0 1.1 1.0

1.0 1.0 1.0

9.0 64.6 151.0

984 5108 11491

960 5640 13100

1.21 1.07 1.03

a

Material specification. bUsing 1H NMR, Mbutyl was determined by the integration of the peak area for butyl-CH3 (δ 0.89), MMA determined by the integration of the peak area for the acrylate peak (δ 5.55), and D determined by the integration of the peak areas (δ −0.10 to −0.25). cUsing 29Si NMR, Mbutyl was determined by the integration of the peak area at δ 7.64, MMA determined by the integration of the peak area at δ 7.40, and D determined by the integration of the peak area at δ −21.21 to δ −22.11.

unchanged macromonomer remaining in the system. However, none of the absorptions corresponding to unreacted methacrylate were evident in in the 1H NMR spectra. The molecular weight of the copolymers was also determined by triple detection GPC (Table 5). From these results, it can be seen that the molecular weight of all the materials prepared is roughly the same, roughly an order of magnitude lower than measured for the pNIPAAm homopolymers that were polymerized in ethyl acetate as previously shown in Table 1. From these results, the average number of PDMS grafts per polymer chain was estimated using eq 4.

NMR spectroscopy (Figure 6), and the molecular weight was determined by triple detection GPC. The feed compositions (Table 4) were selected to provide similar [NIPAAm]/[MA-PDMS] ratios while keeping the MAPDMS in the range of 20−80 wt %. The naming convention used for these samples follows the pattern “xMy”, where x represents the approximate [NIPAAm]/[MA-PDMS] ratio and y represents the approximate Mn of the MA-PDMS macromonomer in kDa. The MA-PDMS wt % determined by 1H NMR in the final polymer composition was always slightly higher than the feed composition, which is likely due to E

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Table 5. Triple Detection GPC Results for Poly(NIPAAm-gPDMS) Initiated Using TEB-MOPA

Figure 6. 1H NMR spectrum of 40M5 in CDCl3, chloroform was used as a reference.

grafts =

sample

Mn(kDa)

ĐM

α

log K

grafts/chain

10M1 20M1 40M1 10M5 20M5 40M5 80M5 20M12 40M12 80M12 160M12

19.3 20.8 25.4 31.2 20.4 31.6 34.2 24.7 23.7 28.6 20.1

3.43 2.9 2.35 3.46 4.04 2.24 1.89 3.1 3.65 2.42 3.67

0.695 0.699 0.714 0.332 0.434 0.527 0.59 0.165 0.293 0.434 0.523

−4.20 −4.17 −4.23 −2.56 −2.96 −3.37 −3.65 −1.55 −2.17 −2.84 −3.32

9.8 7.2 5.3 5.7 3.0 3.4 2.5 1.8 1.5 1.4 0.8

M n,polymer

(113.16 ×

[NIPAAm] [MA‐PDMS]

)+M

n,MA‐PDMS

(4)

The low molecular weight of these copolymers results in a relatively low number of PDMS grafts per molecule specifically for the M12 samples prepared with MA-PDMS12K (Mn = 11 939 Da), which are estimated to have fewer than two grafts per chain, indicating a potential for having pNIPAAm homopolymer in the system. The addition of the PDMS grafts also resulted, unsurprisingly, in an observed increase in dispersity. This increase in dispersity is likely due to a combination of the significant impact a single large MW graft from each macromonomer addition has on the overall MW of the copolymer and the distribution for the number of grafts per chain. The synthesized copolymers were analyzed using DSC to determine the glass transition temperatures of the material, the Tdem, and the miscibility of the polymer grafts within the matrix. The DSC thermograms typically show two glass transition temperatures indicating phase separation of the grafts. For example, 40M5 (Figure 7, middle trace) shows a Tg at −125 and 122 °C corresponding to the grafted PDMS with a Tg of −125 °C31 and the pNIPAAm main chain with a Tg of 140− 143 °C, respectively. In addition, the graft copolymers prepared with the MA-PDMS-12K show crystallization and melting of the PDMS phase near −77 and −45 °C, respectively (see Supporting Information, Figure S1.4−S1.6). Both the MAPDMS-5K and MA-PDMS-12K macromonomers also exhibit crystallization and melting of the PDMS phase; however,

Figure 7. DSC thermograms for poly(NIPAAm-g-MCRM17) showing separate Tgs for the pNIPAAm main chain and PDMS grafts: 20M5 (top trace), 40M5 (middle trace), and 80M5 (bottom trace); second heating scan, 10 °C/min.

copolymers of NIPAAm and MA-PDMS-5K show suppression of these transitions. As expected, the Tg of the pNIPAAm decreases with increasing macromonomer content due to the incorporation of the methacryloyl groups with bulky flexible pendant siloxane moieties into the carbon−carbon polymer main chain (Figure 8). To ensure that such a shift could not be caused by a plasticization effect arising from the presence of any ungrafted MA-PDMS, a set of control measurements were performed with physical blends of pNIPAAm and MA-PDMS-1K

Table 4. Feed and Polymer Compositions for Poly(NIPAAm-g-PDMS) Initiated Using TEB-MOPA feed composition

polymer composition

sample

MA-PDMS (wt %)

[NIPAAm]/[MAPDMS]

[NIPAAm] (mol/L)

[MA-PDMS] (mol/L)

[TEB] (mol/L)

[HOAc] (mol/L)

MA-PDMS (wt %)

[NIPAAm]/[MAPDMS]

10M1 20M1 40M1 10M5 20M5 40M5 80M5 20M12 40M12 80M12 160M12

48.5 33.8 19.0 80.0 66.9 50.1 33.6 83.5 71.7 56.0 38.8

9.2 17.0 39.8 10.0 19.9 40.0 79.3 20.7 41.4 82.4 165.6

0.739 0.963 1.152 0.286 0.474 0.713 0.948 0.236 0.405 0.629 0.898

0.080 0.057 0.029 0.029 0.024 0.018 0.012 0.011 0.10 0.008 0.005

0.081 0.102 0.118 0.033 0.053 0.074 0.097 0.025 0.043 0.062 0.089

0.215 0.290 0.313 0.095 0.134 0.196 0.261 0.070 0.128 0.187 0.309

50.6 34.4 20.7 83.2 66.3 48.6 32.7 86.2 76.0 59.8 48.0

8.5 16.6 33.3 8.1 20.4 42.3 82.4 16.8 33.1 70.4 113.4

F

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

length of NIPAAm in the main chain, would be expected to plateau at the Tdem of the bulk pNIPAAm and decrease as the chain interruptions become more frequent. Although this plot does show the expected plateau, the MA-PDMS-5K and MAPDMS-12K show an increase in the Tdem with increasing macromonomer content. Above about 50 wt % MA-PDMS, the majority phase by volume would be expected to be siliconerich. Since this phase is hydrophobic, this increase observed in the Tdem could in part be attributed to the limited amount of water that can be absorbed into the polymer. Hydration is required to facilitate the main chain conformation changes associated with the coil−globule transition signified by Tdem.5,30 As the amount of macromonomer is increased, the overall hydrophilicity and therefore mobility of the material are decreased, thereby requiring higher temperatures for the transition. The Tdem of aqueous pNIPAAm solutions at polymer concentrations above 35 wt % has been shown to increase, with a sharp increase occurring above 50 wt % along with a sharp decline in the endothermic heat during demixing.34 These reported findings further support the likelihood that decreased swelling as a result of high PDMS content is responsible for the increase in the measured Tdem. The deviation of MA-PDMS-1K copolymers from this behavior could be due to the significantly shorter chain length (average of ∼9 siloxane units per graft) that provides less of a hydrophobic shielding or “umbrella” effect around the main chain. The enthalpy change for the demixing transition (ΔH), measured from integration of DSC endotherms, was normalized to the weight of the copolymer analyzed and plotted as a function of macromonomer content (Figure 10).

Figure 8. Tg of the pNIPAAm phase as a function of MA-PDMS content for macromonomers MA-PDMS-1K (■), MA-PDMS-5K (▲), and MA-PDMS-12K (●); linear regression (R2 = 0.894) drawn using all data points.

homopolymer at the corresponding weight ratios. The DSC thermograms for these samples showed no change in the Tg of the pNIPAAm phase (Supporting Information, Figure S1.7), providing further support that the decrease in Tg is caused by incorporation of the methacryloyl groups into the pNIPAAm main chain. Further, data for the copolymers prepared from all three molecular weight macromonomers show a similar trend indicating that the specific molecular weight of the MA-PDMS macromonomer does not impact the Tg of the pNIPAAm phase. When the data are extrapolated back to zero MA-PDMS content, the Tg of the pNIPAAm phase is predicted to be 139.3 °C, which is in agreement with that measured for the homopolymers (Table 1). The existence of two separate Tgs corresponding to the pNIPAAm and PDMS phases along with this observed decrease in the Tg of the pNIPAAm phase with respect to increasing macromonomer content further supports the graft copolymer structure. Measurement of Thermoresponse. The Tdem of the graft copolymers was measured after swelling in deionized water and the peak of the endotherm was taken as the Tdem. To better understand the effect of the macromonomer on the thermoresponse of the copolymer, Tdem was plotted as a function of macromonomer weight percent (Figure 9A), which shows that as the macromonomer content increases, Tdem also increases. However, previously reported results indicate that hydrophobic monomers decrease the Tdem or LCST.32,33 These reports generally reference the incorporation of conventional (small) hydrophobic or hydrophilic monomers and not macromonomers, in which only the hydrophilicity of the functional end-group likely affects the Tdem. If the change in the Tdem is due to the incorporation of the methacrylate functional group of the macromonomer, then a plot of Tdem as a function of the mole ratio [NIPAAm]/[MAPDMS] (Figure 9B), which is proportional to the average run

Figure 10. Normalized ΔH of graft copolymers swollen in DI water as a function of the MA-PDMS weight percent MA-PDMS-1K (■), MAPDMS-5K (▲), and MA-PDMS-12K (●). The outlier representing the highest loading of MA-PDMS-1K was removed for the shown linear least-squares fit (R2 = 0.939).

Figure 9. Tdem of graft copolymers prepared using MA-PDMS macromonomers MA-PDMS-1K (■), MA-PDMS-5K (▲), and MA-PDMS-12K (●): (A) as a function of MA-PDMS wt %; (B) as a function of [NIPAAm]/[MA-PDMS]. G

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules These data indicate that the thermoresponse is inversely related to the concentration of macromonomer and that the MW of the macromonomer does not have a strong effect. This again shows that the overall hydrophilicity of the matrix may be important to the thermoresponse, as hydrogen bound water is needed for the thermal response to occur, with the shortest side chain macromonomer showing a departure in behavior at the highest level of incorporation. A summary of the thermal data collected from DSC is shown in Table 6. Table 6. DSC Results for Poly(NIPAAm-g-PDMS) Initiated Using TEB-MOPA sample

Tg PDMSa (°C)

Tg NIPAAma (°C)

Tdemb (°C)

norm ΔHb,c (J/g)

10M1 20M1 40M1 10M5 20M5 40M5 80M5 20M12 40M12 80M12 160M12

−112.5 NDe ND d −125.3 −125.5 −130.8 d −125.7 −114.6 −126.1

102.8 114.2 124.1 d 121.9 127.7 132.3 d 125.9 135.1 137.0

ND 27.0 28.5 37.7 33.1 30.9 31.8 ND 36.1 31.5 31.7

ND 13.5 17.9 1.2 3.2 8.1 18.5 ND 1.2 6.6 12.3

Figure 11. AFM images of as-cast poly(NIPAAm-g-PDMS) films; macromonomer molecular weight increases from left to right and weight percent macromonomer decreases from top to bottom.

a

Glass transition temperatures measured by DSC on dry polymer samples. bTdem and ΔH measured by DSC on polymer samples swollen in DI water. cNormalized to the weight of the polymer in the measured sample of polymer swollen in DI water. dSamples were not tested. eND: transition not detected.

the main chain, and deviations from this structure are minimized statistically by the reduced insertion frequency of MA-PDMS. Additionally, intermolecular association of PDMS grafts resulting from the greater “reach” afforded by the longer, flexible 12k siloxane graft arms would facilitate the formation of the expected morphologies. These AFM results show that potentially useful nanophase-separated morphologies can be produced even by the simple, nonliving ambient radical polymerization techniques using air-stable alkylborane initiator complexes.

The aforementioned DSC analysis revealed two separate Tgs indicating phase segregation of the polymer main chain and the PDMS grafts. To confirm this morphology, samples of the graft copolymers were dissolved in propylene glycol monomethyl ether, spin-coated onto silicon wafers, and analyzed as-cast using atomic force microscopy (AFM, Figure 11). From these micrographs, it can be seen that these materials indeed show nanophase separation that is consistent with a copolymer containing immiscible polymer segments even without annealing. For most of these materials, the domain sizes are on the order of 25−30 nm. The copolymers produced with the highest molecular weight MA-PDMS-12K macromonomer resulted in a sufficiently coarse morphology that a lamellar or cocontinuous morphology could be discerned within the AFM resolution limits. This type of microstructure is in reasonable agreement with the morphology reported recently35 in Yshaped block copolymers of PDMS-poly(D,L-lactide) having one PDMS arm and two less flexible (in the sense of being more rigid) poly(D,L-lactide) arms of comparable lengths and volume fractions to that predicted by the feed weight ratio of NIPAAm to MA-PDMS, given that the densities of both homopolymers are within 10% of 1.0 g/cm3. While the present graft copolymers are not ideal in main chain dispersity, the MAPDMS macromonomers and the resulting PDMS grafts themselves are essentially monodisperse (Table 3), the macromonomers having been produced by living anionic methods. For the 80M12 and 160M12 copolymers formed from MA-PDMS-12K, the average frequency of insertion per chain approaches unity (Table 4). In this case, the idealized average structure starts to become blocky, resembling threearmed stars with a single 12k PDMS arm dangling pendant to

■

CONCLUSIONS The potential utility of organoborane-initiated polymerizations under ambient conditions has been further demonstrated and extended to thermoresponsive systems. Although the ambient decomplexation technique using alkylborane−amine complexes does not afford the precision or structural control of controlled radical polymerization techniques, this study demonstrates its practical advantages of process simplicity and versatility because it does not require heat, extreme environmental control, or radiation and can be initiated by simple mixing of the initiator and decomplexer in the presence of oxygen. Successful synthesis of poly(NIPAAm-g-PDMS) polymers from monofunctional macromonomers has been carried out over a range of compositions and graft lengths. The copolymerization of NIPAAm with the MA-PDMS was confirmed by monitoring the reaction kinetics using online 1 H NMR spectroscopy, which showed preferential incorporation of MA-PDMS resulting in compositional drift with respect to conversion. The isolated graft copolymers exhibited nanophase segregation, as expected in copolymers containing immiscible polymer grafts, as observed by AFM. This polymer immiscibility was further confirmed by DSC analysis, which showed that the Tg of the polymer main chain decreased linearly as a function of macromonomer incorporation, while a separate Tg for the PDMS phase was maintained. The impact of H

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

mide) brushes: dependence on grafting density and chain collapse. Langmuir 2011, 27, 8810−8818. (3) Nykaenen, A.; Nuopponen, M.; Laukkanen, A.; Hirvonen, S.-P.; Rytelae, M.; Turunen, O.; Tenhu, H.; Mezzenga, R.; Ikkala, O.; Ruokolainen, J. Phase behavior and temperature-responsive molecular filters based on self-assembly of polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene. Macromolecules 2007, 40, 5827−5834. (4) Kujawa, P.; Aseyev, V.; Tenhu, H.; Winnik, F. M. Temperaturesensitive properties of poly(N-isopropylacrylamide) mesoglobules formed in dilute aqueous solutions heated above their demixing point. Macromolecules 2006, 39 (22), 7686−7693. (5) Liu, R.; Fraylich, M.; Saunders, B. R. Thermoresponsive copolymers: from fundamental studies to applications. Colloid Polym. Sci. 2009, 287, 627−643. (6) Van Durme, K.; Van Assche, G.; Aseyev, V.; Raula, J.; Tenhu, H.; Van Mele, B. Influence of macromolecular architecture on the thermal response rate of amphiphilic copolymers, based on poly(Nisopropylacrylamide) and poly(oxyethylene), in water. Macromolecules 2007, 40, 3765−3772. (7) Zhang, X.-Z.; Zhuo, R.-X. Dynamic properties of temperaturesensitive poly(N-isopropylacrylamide) gel cross-linked through siloxane linkage. Langmuir 2001, 17, 12−16. (8) Kurihara, S.; Minagoshi, A.; Nonaka, T. Preparation of poly(Nisopropylacrylamide)-SiO2 hybrid gels and their thermosensitive properties. J. Appl. Polym. Sci. 1996, 62, 153−159. (9) Mukae, K.; Bae, Y. H.; Okano, T.; Kim, S. W. A new thermosensitive hydrogel: poly(ethylene oxide-dimethylsiloxane-ethylene oxide)/poly(N-isopropylacrylamide) interpenetrating polymer networks. I. Synthesis and characterization. Polym. J. 1990, 22, 206− 217. (10) Liu, L.; Sheardown, H. Glucose permeable poly (dimethyl siloxane) poly (N-isopropyl acrylamide) interpenetrating networks as ophthalmic biomaterials. Biomaterials 2005, 26, 233−244. (11) Hodorog, A. D. R.; Ibanescu, C.; Danu, M.; Simionescu, B. C.; Rocha, L.; Hurduc, N. Thermo-sensitive polymers based on graft polysiloxanes. Polym. Bull. 2012, 69, 579−595. (12) Yildiz, Y.; Uyanik, N.; Erbil, C. Compressive elastic moduli of poly(N-isopropylacrylamide) hydrogels crosslinked with poly(dimethylsiloxane). J. Macromol. Sci., Part A: Pure Appl. Chem. 2006, 43, 1091−1106. (13) Cook, M. T.; Filippov, S. K.; Khutoryanskiy, V. V. Synthesis and solution properties of a temperature-responsive PNIPAM-b-PDMS-bPNIPAM triblock copolymer. Colloid Polym. Sci. 2017, 295 (8), 1351− 1358. (14) Ahn, D.; Wier, K. A.; Mitchell, T. P.; Olney, P. A. Applications of fast, facile, radiation-free radical polymerization techniques enabled by rgoom temperature alkylborane chemistry. ACS Appl. Mater. Interfaces 2015, 7 (43), 23902−23911. (15) Brown, H. C.; Midland, M. M. Initiation rates for autoxidation of trialkylboranes. Effect of a steric factor on the initiation rate. J. Chem. Soc. D 1971, 699−700. (16) Mottus, E. H.; Fields, J. E. Organocarbon and peroxygen polymerization catalysts. US3275611, 1966. (17) Chung, T. C.; Janvikul, W.; Lu, H. L. A novel “stable” radical initiator based on the oxidation adducts of alkyl-9-BBN. J. Am. Chem. Soc. 1996, 118, 705−706. (18) Sonnenschein, M. F.; Webb, S. P.; Rondan, N. G. Amine organoborane complex polymerization initiators and polymerizable compositions. US20020058764A1, 2002. (19) Fujisawa, S.; Imai, Y.; Masuhara, E. Studies on dental self-curing resins. 2. characterization of the various complexes of tri-n-butyl borane as an initiator. Rep. Inst. Med. Dent. Eng. 1969, 3, 64−71. (20) Skoultchi, M. M.; Merlo, N. V. Acrylic adhesive compositions and organoboron initiator system. US5106928A, 1992. (21) Zharov, J. V.; Krasnov, J. N. Polymerizable acrylic compositions with initiator systems based on organoborane amine complexes, and their use as adhesives. WO9522567A1, 1995.

macromonomer incorporation on the thermal properties of the copolymer was readily evident. The Tdem of the materials was also measured, and the compositional range in which thermoresponse is maintained was determined. The impact of low molecular weight PDMS grafts on the Tdem of the copolymer followed the trend previously reported; however, polymer containing higher molecular weight PDMS grafts displayed an opposite trend. At high levels of PDMS, the Tdem values for the copolymer increased and may be related to decreased water uptake.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00252. Detailed experimental data and analysis including DSC thermograms, GPC traces, NMR spectra, and kinetic data tables for reactivity ratio determinations (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.A.). ORCID

Patrick B. Smith: 0000-0003-1593-9451 Bob A. Howell: 0000-0003-1534-4351 Dongchan Ahn: 0000-0002-5938-2192 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Michael R. Reiter for contributing the GPC analysis and Donald V. Eldred for sharing his expertise and instrumentation for the NMR studies. Dow Corning Corporation (now The Dow Chemical Company) is also acknowledged for supporting this research through funding the graduate studies of R.O.H. and providing the resources to complete the experiments in their laboratories.

■

ABBREVIATIONS AFM, atomic force microscope; Da, dalton; EtOAc, ethyl acetate; DSC, differential scanning calorimetry; LCST, lower critical solution temperature; HOAC, acetic acid; HPLC, highperformance liquid chromatography; IPN, interpenetrating network; Tdem, demixing temperature; LCST, lower critical solution temperature; MA-PDMS, monomethacrylate terminated polydimethylsiloxane; Mn, number-average molecular weight; Mw, weight-average molecular weight; NIPAAm, Nisopropylacrylamide; PDMS, polydimethylsiloxane; pNIPAAm, poly(N-isopropylacrylamide); R2, coefficient of determination; RTA, room temperature alkylborane; TEB-MOPA, triethylborane−3-methoxypropylamine complex; Tg, glass transition temperature; THF, tetrahydrofuran; w/w, weight/weight ratio; ĐM, dispersity; δ, chemical shift; ΔH, enthalpy change.

■

REFERENCES

(1) Tobis, J.; Tiller, J. C. Synthesis of chiral and temperatureresponsive silicone-based conetworks. Polym. Prepr. 2006, 47, 1208− 1209. (2) Xue, C.; Yonet-Tanyeri, N.; Brouette, N.; Sferrazza, M.; Braun, P. V.; Leckband, D. E. Protein adsorption on poly(N-isopropylacrylaI

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (22) Ahn, D.; Mohamed, M.; Olney, P. Organosilicon functional boron amine catalyst complexes and curable compositions made therefrom. US7928174B2, 2011. (23) Fedorov, A. V.; Ermoshkin, A. A.; Mejiritski, A.; Neckers, D. C. New method to reduce oxygen surface inhibition by photorelease of boranes from borane/amine complexes. Macromolecules 2007, 40, 3554−3560. (24) Odian, G. Principles of Polymerization; John Wiley & Sons: Hoboken, NJ, 2004. (25) Gramm, S.; Komber, H.; Schmaljohann, D. Copolymerization kinetics of N-isopropylacrylamide and diethylene glycol monomethyl ether mono methacrylate determined by online NMR spectroscopy. J. Polym. Sci., Part A: Polym. Chem. 2005, 43 (1), 142−148. (26) Virtanen, J.; Tenhu, H. Studies on copolymerization of Nisopropylacrylamide and glycidyl methacrylate. J. Polym. Sci., Part A: Polym. Chem. 2001, 39 (21), 3716−3725. (27) Abdollahi, M.; Mehdipour-Ataei, S.; Ziaee, F. Using 1H-NMR spectroscopy for the kinetic study of the in situ solution free-radical copolymerization of styrene and ethyl acrylate. J. Appl. Polym. Sci. 2007, 105 (5), 2588−2597. (28) Jaacks, V. Novel method of determination of reactivity ratios in binary and ternary copolymerizations. Makromol. Chem. 1972, 161, 161−72. (29) Roos, S. G.; Mueller, A. H. E.; Matyjaszewski, K. Copolymerization of n-butyl acrylate with methyl methacrylate and PMMA macromonomers: comparison of reactivity ratios in conventional and atom transfer radical copolymerization. Macromolecules 1999, 32 (25), 8331−8335. (30) Shinoda, H.; Matyjaszewski, K.; Okrasa, L.; Mierzwa, M.; Pakula, T. Structural control of poly(methyl methacrylate)-g-poly(dimethylsiloxane) copolymers using controlled radical polymerization: effect of the molecular structure on morphology and mechanical properties. Macromolecules 2003, 36, 4772−4778. (31) Brook, M. A. Silicones. In Silicon in Organic, Organometallic, and Polymer Chemistry; Wiley-VCH Verlag: 2000; pp 256−308. (32) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Effect of comonomer hydrophilicity and ionization on the lower critical solution temperature of N-isopropylacrylamide copolymers. Macromolecules 1993, 26, 2496−2500. (33) Harmon, M. E.; Kuckling, D.; Pareek, P.; Frank, C. W. Photocross-linkable pNIPAAm copolymers. 4. Effects of copolymerization and cross-linking on the volume-phase transition in constrained hydrogel layers. Langmuir 2003, 19, 10947−10956. (34) Yan, Y.; Huang, L.; Zhang, Q.; Zhou, H. Concentration effect on aggregation and dissolution behavior of poly(N-isopropylacrylamide) in water. J. Appl. Polym. Sci. 2015, 132 (12), 41669/1−41669/8. (35) Minehara, H.; Pitet, L. M.; Kim, S.; Zha, R. H.; Meijer, E. W.; Hawker, C. J. Branched block copolymers for tuning of morphology and feature size in thin film nanolithography. Macromolecules 2016, 49 (6), 2318−2326.

J

DOI: 10.1021/acs.macromol.8b00252 Macromolecules XXXX, XXX, XXX−XXX