Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Process−Structure−Property Relationships for Porous Membranes Formed by Polymerization of Solid Monomer by a Vapor-Phase Initiator Golnaz Dianat, Nareh Movsesian, and Malancha Gupta*
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Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, 925 Bloom Walk, Los Angeles, California 90089, United States ABSTRACT: We determine the mechanism that governs polymerization during a process in which solid monomer is polymerized with a vapor-phase initiator. Gel permeation chromatography data show a bimodal molecular weight distribution at all processing conditions which can be attributed to two different polymerization mechanisms. Smaller chains form by polymerization at the vapor−solid interface, and larger chains form by polymerization within the solid. The monomer mobility and sublimation rate affect the polymerization rate and thereby affect the membrane structure. The molecular weight of the larger chains can be increased by increasing the polymerization temperature and the polymerization time. The ability to vary the polymerization time allows for tuning the solubility of the membranes. The process−structure−property relationships elucidated in this study can enable the fabrication of porous polymer membranes for applications in filtration, textiles, and sensors.
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membranes,29 asymmetric membranes,28 and patterned porous polymer membranes.30 In this work, we systematically study the molecular weight distribution of these membranes for the first time. Formation of the porous polymer membranes can occur by simultaneous or sequential polymerization. In the simultaneous process, the monomer and the initiator free radicals are introduced together in order for the monomer to deposit and polymerize concurrently,19,20 while in the sequential process the monomer is first deposited and subsequently polymerized by the introduction of initiator free radicals.30,31Our work focuses on the sequential method because of the independent control offered over temperature, pressure, and the duration of each step. We show that the sequential process leads to a bimodal polymer distribution due to polymerization at the vapor−solid interface and polymerization within the solid monomer. Polymers with bimodal molecular weight distributions have been shown to balance the mechanical strength and rheological properties of materials.32,33 Methods for initiating the polymerization of solid monomers have been generally limited to using high-energy initiators such as X-rays and electrons; however, activated species can form during these processes, resulting in considerable uncertainties in the mechanistic understanding of these polymerization reactions.34 In addition, polymerization of solid state monomers is reported to be typically slow due to lack of mobility in the solid phase.35,36 For example, Bamford et al. polymerized solid methacrylic acid monomer in solid (4 °C) and near solid (13 °C) states by ultraviolet irradiation and
INTRODUCTION
Porous polymer membranes have many important uses as biomaterials,1,2 sensors,3,4 optical devices,5,6 microreactors,7,8 and filters.9,10 Porous membranes are typically produced by solution-phase methods including phase separation,11,12 cryopolymerization,13,14 solvent casting and particulate leaching,15,16 and copolymer self-assembly.17,18 We have recently shown that we can produce porous polymer membranes by polymerization of solid monomer by a vapor-phase initiator.19,20 In contrast to other fabrication methods, our process is solventless and bottom-up which eliminates solvent compatibility and surface tension issues and offers a green alternative to forming membranes.19 In this paper, we study the process− structure−property relationships that govern our fabrication process which is important for determining polymer properties such as mechanical strength,21,22 solubility,23,24 and crystallization.25,26 In our fabrication process, monomer precursors enter the reactor in the vapor phase in conjunction with initiator molecules. The substrate temperature and monomer pressure are kept below the triple point temperature and pressure of the monomer, respectively, to allow the monomer vapor to deposit as solid pillar-like microstructures. Free radicals that are formed by thermally cleaving the initiator molecules react with the monomer to start polymerization. Unreacted solid monomer is then removed through sublimation, and the porous polymer membrane remains.20,27,28 Larger pores on the order of tens to hundreds of micrometers in magnitude form due to empty spaces between the solid microstructures and smaller pores form on the order of hundreds of nanometers inside the microstructures. Using this method, we have previously shown the fabrication of porous-on-porous © XXXX American Chemical Society
Received: October 13, 2018 Revised: November 17, 2018
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DOI: 10.1021/acs.macromol.8b02201 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Schematic representation of the sequential process for fabricating porous polymer membranes. water to make pH 9 buffer.40 Membranes were dissolved in the GPC eluent and filtered through 0.45 μm filters prior to injection. The injected volume was 100 μL, and the flow rate was 0.5 mL/min. Differential refractive index chromatograms were deconvoluted into two peaks and the area under each peak was calculated by PeakFit v4.12 software using exponentially modified Guassian distribution. The number-averaged molecular weight (Mn) and polydispersity index (PDI) were calculated by a calibration curve equation based on five poly(methacrylic acid), sodium salt, analytical standards with molecular weights of 7000 Da (Polysciences, Inc.), 18700 Da (PSS Polymer Standards Service GmbH), 25000 Da (Polysciences, Inc.), 70000 Da (Polysciences, Inc.), and 350000 Da (Polysciences, Inc.). The membranes formed at a temperature of 10 °C for more than 30 min were found to have high molecular weights (above 450000 Da), which is outside the range of the calibration curve; therefore, the Mn and PDI values were based on a combination of differential refractive index with multiangle light scattering analysis. Three membranes were fabricated for each data point, and the error bars were found from calculating the standard deviation. Scanning electron microscopy (SEM Topcon Aquila) with an accelerating voltage of 20 kV was used to visualize the membranes. A layer of gold was sputter coated onto the membranes prior to SEM imaging to prevent charging. To study the effect of molecular weight on solubility, the membranes were first ground to deconvolute the effect of morphology and mass. Ten milligrams of the ground membranes was dissolved in vials of 10 mL of deionized water and in vials of 10 mL of pH 9 buffer. To study the effect of morphology on solubility, the membranes were immersed in Petri dishes containing a constant volume (25 mL) of pH 9 buffer.
found that conversion reaches 10% after 65 and 30 h in the solid and near solid states, respectively.35 In contrast, we utilize a low-energy process using a thermally cleavable vapor phase initiator that can easily diffuse in the solid monomer microstructures. This process operates at mild reactor conditions with the ability to retain the full chemical functionality of the precursors.37−39 Here we demonstrate that the molecular weight distribution of the polymer membranes formed by our technique can be tuned by changing the polymerization temperature and time, which can lead to changes in the membrane dissolution rate.
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EXPERIMENTAL SECTION
Methacrylic acid (MAA) (Aldrich, 99%), tert-butyl peroxide (TBPO) (Aldrich, 98%), sodium phosphate dibasic (Aldrich, ≥99.0%), and sodium azide (Aldrich, ≥99.5%) were used as received without further purification. Silicon wafers with 3 × 3 cm2 dimension were placed on top of a thermoelectric cooler (TE Technology) which was located on the stage of a custom-designed initiated chemical vapor deposition (iCVD) vacuum reactor (GVD Corporation, 250 mm diameter, 48 mm height). The temperature of the thermoelectric cooler (TEC) was regulated using an adjustable dc power supply (Volteq HY3010D). The stage temperature was maintained at 10 °C using a recirculating chiller (Thermo Scientific Haake A25). A rotary vane vacuum pump (Edwards E2M40) and a throttle valve (MKS 153D) were used to control the pressure of the reactor. To synthesize the porous poly(methacrylic acid) PMAA membranes, the TEC was maintained at a temperature of −10 °C during monomer deposition. TBPO was introduced into the chamber at 0.6 sccm to maintain the pressure at 650 mTorr. The MAA monomer was then introduced into the chamber at a flow rate of 0.3 sccm for 5 min for all the experiments. Next, MAA flow was stopped, and TBPO continued flowing into the reactor at a reactor pressure of 650 mTorr at 0.6 sccm. To study the effect of the polymerization temperature, the TEC temperature was set to −20, −10, 0, and 10 °C immediately after monomer deposition and before starting the polymerization. Polymerization was started by resistively heating the filament array to 240 °C to decompose the initiator molecules into free radicals. The filament was then turned off after polymerization, and the flow of TBPO was halted. The TEC temperature was then kept at 5 °C and slowly increased to 14 °C to allow the unreacted MAA to sublimate and leave the system, which was confirmed by the reactor returning to a base pressure of 18 mTorr. During the porous PMAA fabrication process, a silicon wafer was placed on the stage next to the TEC and the thickness of the polymer deposited onto it was monitored by an in situ 633 nm helium−neon laser interferometer (Industrial Fiber Optics). A gel permeation chromatography (GPC) system equipped with a HPLC pump (Agilent 1200 series), a refractive index detector (Wyatt Optilab rEX), a multiangle light scattering detector (Wyatt DAWN HELEOS), and two Agilent columns (PL aquagel-OH MIXED-M) in series was used to measure the molecular weight distribution of the polymer membranes. The GPC eluent was made by dissolving 0.1 M sodium phosphate dibasic and 200 ppm sodium azide in deionized
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RESULTS AND DISCUSSION Figure 1 shows a schematic of the sequential deposition process to fabricate porous polymer membranes. A wafer is placed on a thermoelectric cooler (TEC) that is set at −10 °C. The TEC is located on the reactor stage which is cooled to 10 °C. First, the initiator is introduced into the reactor to build up and maintain the reactor pressure. Then the monomer is flown into the reactor for 5 min for all the experiments to keep the initial monomer concentration (∼300 mg) constant. We use methacrylic acid (MAA) as the monomer due to its relatively high freezing point (15 °C) and the useful properties of the resulting poly(methacrylic acid) (PMAA) membranes such as pH-responsiveness.41,42 The flow of the monomer is then halted, and the filament is turned on to cleave the initiator molecules into free radicals to start the polymerization process. In this study, the polymerization temperature is systematically varied by changing the TEC temperature to −20, −10, 0, or 10 °C right before turning on the filament. The polymerization time is varied by keeping the filament on from 5 to 240 min. After the polymerization process, unreacted solid monomer is sublimated. We first investigated the effect of polymerization time on the morphology of the porous membranes. Figure 2a shows that the structures formed after 5 min of polymerization at a B
DOI: 10.1021/acs.macromol.8b02201 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
polymer chains produced at each condition indicate that the number of polymer chains increases with increasing time for the low molecular weight fraction since initiator radicals are constantly introduced at the vapor−solid interface, whereas the number of polymer chains for the high molecular weight fraction stays relatively constant which suggests the consumption of solid-state monomer into the growth of existing polymer chains. The formation of longer chains with lower PDIs can be attributed to the lower initiator concentration inside the pillars, resulting in lower termination rates. Our hypothesis is that the polymer formed at the vapor−solid interface acts as a diffusion barrier for the free radical initiators. Therefore, termination of the growing radical chains within the solid is slowed due to a lower initiator concentration. To understand the effect of polymerization temperature on the polymer morphology and the molecular weight distribution, we set the TEC temperature to −20, −10, 0, and 10 °C during the polymerization step while keeping all other processing parameters constant. The SEM images in Figure 3 show that the densified porous structures that form at the silicon wafer boundary thicken as the temperature is increased even though the total membrane thickness remains unchanged (∼650 μm). The increase in the thickness of the densified porous structure is likely due to higher sublimation rates at higher polymerization temperatures. To confirm our hypothesis, we placed a silicon wafer on the stage at 10 °C next to the TEC to monitor the sublimation process. During the polymerization on the TEC, monomer sublimating from the porous membrane adsorbs onto the silicon wafer in the vapor phase and polymerizes to form a dense pinhole-free polymer film. The thickness of this dense film was measured by in situ interferometry at all polymerization temperatures (−20, −10, 0, and 10 °C) for a constant polymerization time of 30 min. The thickness of the dense polymer films increased from approximately 50 to 500 nm as the temperature increased from −20 to 10 °C, confirming that the higher polymerization temperature increases the sublimation rate of the solid monomer and thus results in higher monomer adsorption and polymerization on the silicon wafer placed on the stage. As shown in Figure 3, the pillar structure of the membranes formed at −20 °C does not have visible micrometer scale pores while the morphology of the pillars formed at −10 and 0 °C are similar and sponge-like with small-scale pores within the pillars. The higher sublimation rate at higher polymerization temperatures likely leads to the formation of coarser pores within the pillars. This data are consistent with recent studies on the effect of sublimation rate on the pore size of porous particles formed by vapor deposition of poly(p-xylylene)s on sublimating ice particles.43 The pillars formed at 10 °C are partially covered with a dense film which is likely due to condensation and polymerization of the monomer at the surface of the pillars because of the higher local monomer
Figure 2. Angled-view and cross-sectional SEM images of porous membranes polymerized for (a) 5 and (b) 30 min at a polymerization temperature of −10 °C.
temperature of −10 °C are tilted which is likely due to their lack of mechanical strength, resulting in structural collapse during the sublimation process. Figure 2b shows that a densified porous structure formed at the silicon wafer boundary as the polymerization time was increased from 5 to 30 min. This densified porous structure adds to the mechanical strength of the membranes, and therefore our remaining experiments focused on membranes formed at polymerization times above 30 min. To systematically understand the effect of polymerization time on the molecular weight distribution, we formed membranes at polymerization times of 30, 60, 120, and 240 min at a polymerization temperature of −10 °C. The membranes were dissolved in pH 9 buffer, and the molecular weight distributions were measured using GPC. The GPC data in Table 1 show that the membranes have a bimodal molecular weight distribution for all polymerization times, which indicates the presence of two types of polymerization mechanisms. Our hypothesis is that the polymerization starts at the vapor−solid interface and forms lower molecular weight chains because the higher initiator concentration at the interface results in a faster termination rate. The higher molecular weight chains are likely formed by diffusion of the initiator free radicals inside the solid monomer pillars. The GPC data show that the Mn of the shorter chains does not significantly vary with time while the Mn of the longer chains increases. The data also show that the total mass of polymer increases with time. The mass of the lower molecular weight fraction increases more drastically than the higher molecular weight fraction. Comparisons among the estimated number of
Table 1. Effect of Polymerization Time on the Molecular Weight Distribution and the Total Mass of Polymer Formed at a Polymerization Temperature of −10 °C peak 1 time (min)
temp (°C)
30 60 120 240
−10 −10 −10 −10
total mass (mg) 8.6 14.9 21.2 33.7
± ± ± ±
0.8 3.9 1.1 3.6
Mn (kDa) 133.1 176.5 256.5 335.0
± ± ± ±
17.2 28.2 34.8 60.8
mass (mg)
± ± ± ±
2.2 2.9 4.0 4.6
2.1 1.9 1.7 1.5 C
peak 2
PDI 0.3 0.2 0.1 0.1
Mn (kDa) 11.9 9.2 8.9 9.0
± ± ± ±
5.5 2.2 2.3 2.0
PDI
mass (mg)
± ± ± ±
6.4 12.0 17.2 29.1
2.3 3.0 2.7 2.4
0.4 0.6 0.2 0.2
DOI: 10.1021/acs.macromol.8b02201 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. Cross-sectional and zoomed-in SEM images of the membranes formed at different polymerization temperatures.
Table 2. Effect of Polymerization Temperature on the Molecular Weight Distribution and the Total Mass of Polymer peak 1 time (min)
temp (°C)
30 30 30 30
−20 −10 0 10
Mn (kDa)
total mass (mg) 5.9 8.6 11.7 18.3
± ± ± ±
1.4 0.8 3.1 1.5
69.1 133.1 243.8 402.2
± ± ± ±
10.4 17.2 36.7 59.3
peak 2
PDI
mass (mg)
± ± ± ±
2.7 2.2 2.3 2.2
2.4 2.1 1.8 1.7
0.2 0.3 0.1 0.1
Mn (kDa) 10.1 11.9 16.6 17.5
± ± ± ±
1.6 5.5 2.1 2.5
PDI
mass (mg)
± ± ± ±
3.2 6.4 9.4 16.1
1.6 2.3 2.4 3.0
0.1 0.4 0.3 0.3
Table 3. Effect of Initial Monomer Concentration on the Molecular Weight Distribution and the Total Mass of the Polymer peak 1
peak 2
deposition (min)
time (min)
temp (°C)
total mass (mg)
Mn (kDa)
PDI
mass (mg)
Mn (kDa)
PDI
mass (mg)
2 5
30 30
−10 −10
4.7 8.6
128.6 ± 3.7 133.1 ± 17.2
1.7 ± 0.6 2.1 ± 0.3
1.8 2.3
11.7 ± 5.7 11.9 ± 5.5
2.7 ± 1.4 2.3 ± 0.4
2.9 6.4
sublimation rate is the dominant factor in the polymerization at the vapor−solid interface. To further study the polymerization mechanism, we synthesized membranes at a lower initial monomer concentration where monomer was deposited for 2 min instead of 5 min as in all other cases and polymerized for 30 min at −10 °C. The GPC data for the membranes formed from 2 min of monomer deposition shows similar molecular weights for both the shorter and longer chains as the membranes formed from 5 min of monomer deposition (Table 3). Therefore, it can be concluded that the polymerization rate is not limited by the initial monomer concentration under these conditions. As shown in the SEM images in Figure 4, the membranes that were deposited with a higher initial monomer concentration (5 min) are thicker and therefore have a higher vapor−solid interfacial area which results in a greater mass of shorter polymer chains. To test our hypothesis that the lower molecular weight chains form by polymerization at the vapor−solid interface, we studied the molecular weight of the dense films that formed on the silicon wafer placed on the stage next to the TEC during a
partial pressure and polymerization temperature close to the freezing point of MAA. As shown in Table 2, there is a bimodal distribution of molecular weights at each polymerization temperature. The Mn of the lower molecular weight chains has a moderate increase while the Mn of the higher molecular weight chains increases drastically with increasing polymerization temperature due to the increased mobility of the monomer molecules, leading to higher propagation rates. The mass of the higher molecular weight chains stays constant, while the mass of the shorter chains increases with increasing temperature due to the high concentration of the free radicals at the vapor−solid interface and the higher sublimation rates at higher temperatures. This results in an increased monomer conversion from ∼2% (5.9 mg) at a polymerization temperature of −20 °C to ∼6% (18.3 mg) at 10 °C. (Table 2) These data are consistent with the results shown by Bamford et al. on the polymerization of solid MAA by ultraviolet irradiation which shows slower polymerization rates at lower temperatures.35 Based on the data, the polymerization of the solid monomer is dominated by the reaction kinetics governed by temperature while the monomer D
DOI: 10.1021/acs.macromol.8b02201 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
similar in magnitude to the Mn of the lower molecular weight fraction (17.5 × 103 g/mol) found in the porous membrane fabricated on the TEC in the same deposition. This experiment was repeated for polymerization times of 75 and 120 min and the Mn of the dense films found to be 9.5 × 103 and 11.8 × 103 g/mol, respectively. Increasing the time did not increase the molecular weight, which is consistent with the trends observed for the low molecular weight fraction of the porous membranes. To further confirm the effect of time and temperature on the molecular weight distribution, we formed membranes at 10 °C for 30, 75, 120, and 240 min. The mass of the low molecular weight fraction increases more rapidly than the mass of the high molecular weight fraction, which is consistent with the data in Table 1. For each of the polymerization times, the Mn of the higher molecular weight fraction and the total polymer yield are greater at 10 °C than −10 °C due to the higher mobility and sublimation rate, respectively (Table 4). The Mn of the low molecular weight fraction is slightly higher at 10 °C (Table 4) compared to the Mn of the low molecular weight fraction at −10 °C (Table 1) due to faster kinetics and a higher monomer concentration likely caused by a higher sublimation rate at this temperature. We further observe that the monomer sublimation rate during the polymerization step reaches steady state between 75 and 90 min of polymerization by monitoring the dense film deposition via interferometry. Thus, there is a decrease in the mass growth rate of the low molecular weight fraction over time which indicates that the sublimation rate is the dominant factor in the polymerization at the vapor−solid interface. Mn of the higher molecular weight fraction for polymerization times of 75 min and above are on the order of 1000 kDa, and multiangle light scattering was used to confirm these high molecular weights. The ability to tune the solubility of the porous membranes is important for applications such as water filtration and drug delivery. As shown above, increasing the time and temperature of polymerization increases the molecular weight of the polymer, which can be used to decrease solubility.23 To study the dissolution behavior of the membranes, three different porous membranes were fabricated at a polymerization time of 30, 75, and 120 min and a polymerization temperature of 10 °C. The membranes were first ground to minimize the effect of the membrane morphology on the dissolution rate, and then 10 mg of each membrane was immersed in 10 mL of deionized water. As shown in Table 5, the dissolution rate of the porous membranes decreased by increasing the polymerization time. To further investigate the dissolution behavior, the ground membranes were also dissolved in pH 9 buffer, and the same trend was observed with increasing polymerization time. To elucidate the effect of morphology on the dissolution rate, the intact porous membranes that were polymerized for 30 and 120 min at a polymerization temperature of 10 °C with a total mass of 19.5
Figure 4. Angled-view and cross-sectional SEM images of porous membranes that were formed by depositing monomer for (a) 2 and (b) 5 min and then polymerizing for 30 min at a temperature of −10 °C.
polymerization temperature of 10 °C. The high polymerization temperature resulted in a high monomer sublimation rate from the TEC and the sublimated monomer that adsorbed onto the silicon wafer on the stage polymerized to form a dense transparent film. The GPC data in Figure 5 demonstrate that
Figure 5. GPC chromatograms of the porous membrane synthesized by 30 min of polymerization at 10 °C and the dense films deposited on the silicon wafer on the stage for 30, 75, and 120 min at a temperature of 10 °C. Each chromatogram is normalized with respect to the area under the corresponding curve.
the molecular weight of the dense film, polymerized for 30 min, has one peak with a Mn of 11.6 × 103 g/mol, which is
Table 4. Effect of Polymerization Time on the Molecular Weight Distribution and the Total Mass of Polymer at 10 °C peak 1 time (min)
temp (°C)
30 75 120 240
10 10 10 10
total mass (mg) 18.3 27.8 30.3 38.2
± ± ± ±
1.5 3.1 8.0 0.8
Mn (kDa) 402.2 1735.7 1255.7 1349.1
± ± ± ±
59.3 591.7 333.6 220.1
mass (mg)
± ± ± ±
2.2 4.3 5.0 8.9
1.7 1.5 1.2 1.3 E
peak 2
PDI 0.1 0.5 0.2 0.1
Mn (kDa) 17.5 17.1 17.8 20.3
± ± ± ±
2.5 1.4 2.9 1.1
PDI
mass (mg)
± ± ± ±
16.1 23.5 25.3 29.3
3.0 2.6 3.1 3.5
0.3 0.4 0.3 0.5
DOI: 10.1021/acs.macromol.8b02201 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 5. Effect of Molecular Weight on the Dissolution Rate of Porous Polymer peak 1
peak 2
dissolution time (min)
time (min)
temp (°C)
Mn (kDa)
% mass
Mn (kDa)
% mass
DI water
pH 9 buffer
30 75 120
10 10 10
402.2 ± 59.3 1735.7 ± 591.7 1255.7 ± 333.6
12.0 14.6 15.1
17.5 ± 2.5 17.1 ± 1.4 17.8 ± 2.9
88.0 85.4 84.9
12 65 120
1 7 22
Figure 6. Dissolution behavior in 25 mL of pH 9 buffer of the porous membranes polymerized at 10 °C for (a) 30 and (b) 120 min.
and 23.5 mg, respectively, were each immersed in the same volume (25 mL) of pH 9 buffer. The membrane formed at a shorter polymerization time dissolved within 1 min in the solvent while the membrane formed at a longer time required over 15 min to dissolve completely (Figure 6).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation CAREER Award CMMI-1252651. We thank Dr. Shuxing Li and the USC NanoBiophysics Core Facility for help with the gel permeation chromatography experiments.
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CONCLUSION We have shown that polymerization of solid monomer with a vapor phase initiator results in a bimodal molecular weight distribution. Polymerization at the vapor−solid interface leads to the formation of shorter chains and polymerization within the solid monomer structures leads to higher molecular weight chains. The molecular weight of the longer chains increases with time, while the molecular weight of the shorter chains stays relatively constant due to the high concentration of initiator radicals at the vapor−solid interface. Higher polymerization temperatures affect the molecular weight distribution of the membranes by altering the mobility of the solid monomer inside the pillars, resulting in an increase in the molecular weight. A densified porous structure forms at the silicon wafer boundary and thickens as a result of the higher sublimation rate at higher temperatures. We show that we can control the molecular weight distribution with time and temperature to tune the dissolution rate of the membranes. These findings set important guidelines for the fabrication of next-generation porous materials with tailored structures and properties for practical applications in separations, textiles, and sensors.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (M.G.). ORCID
Malancha Gupta: 0000-0002-6828-7445 F
DOI: 10.1021/acs.macromol.8b02201 Macromolecules XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.macromol.8b02201 Macromolecules XXXX, XXX, XXX−XXX
