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Supporting Information

Polymer Microspheres Prepared by Water-Borne Thiol-Ene

Suspension Photopolymerization

Olivia Z Durham,† Sitaraman Krishnan,

‡ and Devon A. Shipp

†,*

† Department of Chemistry & Biomolecular Science and Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5810, United States. * dshipp@clarkson.edu

‡ Department of Chemical & Biomolecular Engineering and Center of Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5705, United States.

EXPERIMENTAL

Materials. 1,3,5-Triallyl-1,3,5-triazine-2,4,6 (1N,3H,5H)-trione (TTT, CAS no. 1025-15-6),

pentaerythritol tetrakis(3-mercaptopropionate) (PETMP, CAS no. 7575-23-7), 1-

hydroxycyclohexyl phenyl ketone (CAS no. 947-19-3), and sodium dodecyl sulfate (SDS, CAS

no. 151-21-3) were all obtained from Sigma-Aldrich and used without further purification.

Chloroform and toluene were obtained from VWR Scientific and used without further purifica-

tion.

Polymerization Setups. “Small Scale” reactions were done in 20 ml scintillation vials with

magnetic stirring throughout polymerization. “Large Scale” reactions were done in a 250 ml

round bottom flask with a 24/40 neck. These reactions were mixed with overhead stirring with a

7.2 cm blade or propeller throughout polymerization. “Sonication” reactions used “large scale”

reaction procedure but sonication (without irradiation) was performed in a ~ 400 ml temperature

controlled flask. After sonication, the reaction mixture was transferred to a 250 ml round bottom

flask and mixed using the same overhead stirrer as the “Large Scale” reactions. In each

polymerization approach, an Oriel Instruments, model 68811, 500W mercury xenon arc lamp

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(intensity ~90mW/cm2, as measured by a Dymax Corp. Accu-Cal-30 intensity meter) was used.

Synthesis of Micron-Sized Thiol-Ene Polymer Particles using Mechanical Stirring. The

general suspension photopolymerization mechanism involves the mechanical mixing of two sep-

arate phases, an organic phase and an aqueous phase. The organic phase consists of an approxi-

mately 1:1 mole ratio of thiol and ene functionality along with a 1:1 volume ratio of the mono-

mer blend to solvent. The photoinitiator is soluble in the organic phase, but reactions examining

placement of the initiator in the organic phase as well as the aqueous phase have yielded the

same observations and final polymer product. The aqueous phase consists of a surfactant solu-

tion of a desired concentration. The organic phase was added to the aqueous solution and the

reaction mixture is stirred for approximately 5-10 minutes and then is cured in the presence of

UV light (365 nm wavelength) for another 5-10 minutes with continuous stirring. This general

process was used for reactions on 10 g scale (“small scale”) and 88 g scale (“large scale”). Small

scale reactions have used a magnetic stir plate and a 5 minute reaction time whereas large scale

reactions have used an overhead stirrer and 10 minute mixing and curing. Table SI-1 gives the

recipe for the large scale polymerization reaction.

Table SI-1. General recipe for water-borne thiol-ene suspension photopolymerization

Mass (g) Overall mass

%

Organic phase

TTT 3.037 (36.5 mmol. vinyl group) 3.45

PETMP 4.454 (36.5 mmol. thiol group) 5.07

1-Hydroxylcyclohexyl phenyl ke-

tone 0.015 (0.073 mmol.) 0.02

Toluene 5.375 (ca. 50 vol % of organic phase) 6.11

Aqueous phase

SDS 3.75 (13.0 mmol; ca. 5 wt % of aq.

phase) 4.26

Water 71.3 81.09

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Synthesis of Sub-Micron Thiol-Ene Polymer Particles using Sonication. The procedure for

the synthesis of sub-micron polymer particles followed the same procedure as large-scale photo-

polymerization reactions described above with the following two changes. The addition of hex-

adecane (0.047 g, 0.36 wt.% of the organic phase) to the reaction mixture and sonication prior to

photopolymerization by an Ace Glass sonic horn (Model GEX600, 20 Hz, 600 W) for 15-60

minutes. The reaction solution was sonicated at 35ºC with an amplitude of 38 %, pulse on for

1.5 s, and pulse off for 2.0 s at a frequency of 20 kHz. The reaction was also stirred with a small

stir bar during sonication. Twenty minutes after the sonication process, the reaction mixture was

irradiated for 10 minutes with overhead stirring.

Characterization. Analysis of products was performed using an Olympus optical microscope,

where samples were prepared by simply air-drying or drying in a vacuum oven. Polymer parti-

cles were also analyzed using field emission scanning electron microscopy (FE-SEM) using a

JEOL JSM 6300 instrument. For electron microscopic analysis, product material was placed on

an aluminum stub and dried in a vacuum oven until all solvent was removed. Prior to analysis,

samples were sputter-coated with approximately 6 nm thick Au/Pd layer. Particle size distribu-

tions in the aqueous dispersions were measured on a ZetaPALS instrument (Brookhaven Instru-

ments). Differential scanning calorimetry (DSC) was performed on a TA Instruments Q100 in-

strument, with a heating rate of 10ºC/min.

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RESULTS

1. Effect of Homogenization Energy (Mechanical Shear)

A small-scale reaction was conducted using SDS as an emulsifier in order to observe the effect

of homogenization energy on particle size and particle size distribution. Homogenization energy

was varied by simply altering the stirring speed of the magnetic stir bar. Both images shown in

Figure SI-1 were taken at 10× magnification using an optical microscope. It can be seen in these

images that higher homogenization energy gives smaller particles.

Figure SI-1. Particles formed from small-scale reactions using magnetic stirring in order to ex-

amine the effect of homogenization energy of particle size and particle size distribution. (A) Par-

ticles obtained using low homogenization energy (slower rotational speed of magnetic stirrer).

(B) Particles obtained using higher homogenization energy (faster stirring rate). The homogeni-

zation energy of reaction (B) was twice that as demonstrated in reaction (A). Both reactions

used the recipe shown in Table SI-1.

2. Variation of Solvent and Solvent Concentration

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Two reaction systems, each with different solvent, were examined in order to determine the ef-

fect of solvent concentration. The solvents used were chloroform and toluene. Other common

solvents (such as diethyl ether, ethyl acetate, hexane, cyclohexane, and 1,4-dioxane) resulted in

colloidally unstable latexes and significant coagulum formation. Figure SI-2 shows particles

formed using chloroform as the solvent, with the ratio of monomer:solvent equal to about 1:1,

2:1 and 4:1, for images SI-2(A), (B) and (C), respectively. Clearly, the ratio of solvent to the

monomer affects the particle size and particle size distribution, and as the amount of solvent is

reduced the particles appear to be larger. A similar trend is observable in the images shown Fig-

ure SI-3(A), (B) and (C), where toluene was used as the solvent. No significant difference in the

particles sizes was noticed between the two different solvents when prepared using comparable

procedures. The decrease in the particle size with an increase in the amount of solvent in the rec-

ipe is attributed to the lower viscosity of the dispersed phase, leading to the formation of smaller

microdroplets during emulsification.

Figure SI-2. Optical microscopy images of polymer particles made using chloroform as solvent.

(A), (B), and (C) describe the of 1:1, 2:1, and 4:1 volume ratio of monomers to solvent, respec-

tively. All reactions used the recipe shown in Table SI-1, and identical magnetic stirring reaction

conditions.

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Figure SI-3. Optical microscopy images of polymer particles made using toluene as solvent.

(A), (B), and (C) describe the of 1:1, 2:1, and 4:1 volume ratio of monomers to solvent, respec-

tively. All reactions used the recipe shown in Table SI-1, and identical magnetic stirring reaction

conditions.

3. Variation in Surfactant Concentration

Figure SI-4 shows particles that were made using 5 wt.% and 10 wt.% surfactant (in the aqueous

phase). These images show that at higher concentrations of surfactant, smaller and more uni-

formly sized polymer particles are obtained.

Figure SI-4. Particles from large-scale reactions using (A) 10 wt% SDS or (B) 5 wt% SDS.

Both reactions used the recipe shown in Table SI-1, and a 1:1 volume of monomers:toluene.

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4. Analysis of Thermal Properties

Differential scanning calorimetry (DSC) traces for the thermal analysis of a bulk reaction [Figure

SI-5(A)] and material obtained from a water-borne polymerization [Figure SI-5(B)] show that

virtually identical glass transitions are obtained, with the glass transition temperatures (Tg) esti-

mated to be about 3oC and –1

oC, respectively.

Figure SI-5. DSC traces of products produced from a (A) bulk reaction and a (B) water-borne

reaction.