Something went wrong. Try again.
This repository has no description
Something went wrong. Try again.
PhD-Dissertation Chapter3.typ
32 kB · 603 lines
at main
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549550551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602603604*#align(center)[#smallcaps[@highly-crosslinked-ultra-hard-networks-from-polymer-combs[Chapter]]]*= #smallcaps[Highly crosslinked, ultra-hard networks from polymer combs]<highly-crosslinked-ultra-hard-networks-from-polymer-combs>#align(center)[Publication in preparation]
== Abstract<abstract>Comb polymers are branched macromolecules used as compatibilizers,surface modifiers, and, when polymerized, as strong networks. Combpolymers are typically synthesized to contain different backbone andsidechain chemistries, which can result in multifunctional reactivityand strong phase separation from different polymer domains. Here, weexplored designing comb polymers that could result in high modulus,tough networks by synthesizing polymers with glycidyl methacrylatesidechains capable of participating in dense crosslinking reactions. Wesynthesized several comb polymers containing homopolymer, block, orrandom copolymer sidechains, and compared their mechanical propertiesand network structures to their linear counterparts. Comb polymers withrandom copolymer side chains had superior toughness, even compared totheir linear counterparts. I hypothesize that statistical distributionof monomers along the grafts break up crosslinking sites, disfavoringintramolecular crosslinks and favor intermolecular crosslinks. I expectthese polymers to be useful for multi-functional coatings and asadhesives with high modulus and toughness.
== Introduction<introduction>Comb polymers are macromolecules with polymers grafted onto a mainpolymer backbone. They are increasingly popular for their lowviscosity,@manson1958 ease of side-chain functionalization, quickgelation times,@reynolds2020 complex phase behavior,@jiang2009 and largeparameter design space.@zhang2016 Comb polymers have applicationsranging from next-generation compatibilizers between immiscible polymerphases,@dong2015 tough networks and adhesives,@zhang2016a@ohnsorg2024anionic exchange resins,@zhu2016 viscosity modifiers,@2009 tough resinswith crystalline domains.@claesson2004@lorenzana2024 The low viscosityof these polymers and polymer solutions offers the additional benefit ofrequiring less energy to process.@kong2021
Previously, our group has created mechanosensitive networks from combcopolymers with reactive monomers along the main backbone.@lorenzana2024Using poly(ethylene glycol) (PEG) grafts as molecular shields blockingthe reactive sites on the main backbone, I produced crosslinkedmaterials upon application of ultrasound. In that system, the materialmass was dominated by inert PEG side chains. This resulted in lowoverall density of crosslinks, limiting the maximum modulus of thepolymers. To address this limitation, here I have developed a suite ofreactive comb polymers, where each sidechain is itself reactive andcrosslinkable, significantly increasing the potential for new crosslinksand high modulus networks. I hypothesized that GMA containing graftswould allow for an increase in crosslinking density while stillproviding ultrasound activated crosslinking. This work was focused onthe mechanical properties of these graft polymers.
Most comb polymers used to create networks have crosslinking sites alongthe main polymer backbone, while the grafts do not contribute to themodulus. In contrast, I synthesized a well-defined methacrylicpolyinitiator (pBIEM), where each monomer on the chain contains apendent alkyl bromine capable of initiating atom transfer radicalpolymerization (ATRP). Homo- and co-polymers of GMA and BMA were thengrown off the backbone using a \"grafting from\" approach to preparedensely grafted comb polymers. I then crosslinked the polymer combs toform networks with elastic moduli of 1 GPa and with toughnesses of500kPa. I anticipate that this work will provide strategies forpreparing high modulus, tough comb polymer coatings and adhesives.
== Materials and methods<materials-and-methods>=== Chemical sourcing<chemical-sourcing>Materials were purchased from Sigma-Aldrich unless otherwise mentioned.2-Hydroxyethyl methacrylate (HEMA, 98%), $alpha$-bromoisobutyryl bromide(99%), pyridine (anhydrous, 99%), dichloromethane (DCM, anhydrous, 99%),dioxane (99%), CuBr#sub[2] (99%), 1,5,7-triazabicyclo\[4.4.0\]dec-5-ene(TBD, 98%), ethyl $alpha$-bromophenylacetate (EBPA, 97%),N,N,N',N”,N”-pentamethyldiethylenetriamine (PMDETA, 99%),1,1,3,3-tetramethylguanidine (TMG, 99%), and azobisisobutyronitrile(AIBN, 99%), were used as received. Glycidyl methacrylate (GMA, 99%),butyl methacrylate (BMA, 99%) were passed through a column of basicalumina to remove inhibitors. CuBr (99.9%) was purified by stirring inglacial acetic acid. Tris(2-pyridylmethyl)amine (TPMA, 98%), tris2-(dimethylamino)ethyl amine (Me6TREN, 98%), cyclopentylmethyl ether(CPME, 99%), and 2-cyano-2-propyl dodecyl trithiocarbonate (CPDT, 97%)was purchased from TCI (Tokyo, Japan) and used as received. Methanol(99%), toluene (99%), dimethylformamide (DMF, 99%), isopropanol (99%),calcium chloride (99%) diethyl ether (anhydrous, 99%), tetrahydrofuran(THF, 99%), HCl (concentrated), reduced iron powder, copper turnings,basic alumina, and neutral alumina were purchased from Thermo Fisher(Waltham, MA) and used as received.
=== 2-(2-Bromoisobutyryloxy)ethyl methacrylate (BIEM) synthesis<bromoisobutyryloxyethyl-methacrylate-biem-synthesis>BIEM was synthesized according to a previously publishedprotocol.@graff2015 Briefly, to a 250 mL three-necked round bottom flask(RBF), 50 mL of anhydrous DCM, 10 mL of HEMA (0.1 mol), and 10 mL ofanhydrous pyridine (0.12 mol), and a magnetic stir bar are added. TheRBF is then lowered into an ice bath and stirred for ten minutes.Meanwhile, a drying tube filled with calcium chloride, an additionfunnel attached to a bubbler filled with silicone oil, and a glassadaptor attached to a nitrogen line are attached to each neck of theflask. 7 mL of $alpha$-bromoisobutyryl bromide (0.1 mol) is dissolvedinto 20 mL of anhydrous DCM and added to the addition funnel. The$alpha$-bromoisobutyryl bromide solution is added dropwise to the RBFover 15 minutes, after which the reaction is allowed to stir in the icebath for a further 45 minutes, followed by removal from the ice bath.After stirring at room temperature for two hours, the crude product isfiltered through cotton to remove pyridine salts and then concentratedunder vacuum. The product is purified by flash chromatography oversilica gel with chloroform, yielding a clear colorless oil afterevaporation of the chloroform.
=== Copper activation and stirbar cleaning<copper-activation-and-stirbar-cleaning>Copper turnings were activated by tying around a magnetic stir barfollowed by immersion in conc. HCl for 15 minutes to remove copperoxides followed by rinsing with methanol. Stir bars were cleaned ofresidual metals by submerging in concentrated HCl overnight, followed bywashing with DI water and ethanol.
=== CuBr purification<cubr-purification>To a 500 mL beaker was added 250 mL of glacial acetic acid followed by25 g of CuBr and a magnetic stirbar. The beaker was covered withparafilm and allowed to stir vigorously for two days. After two days,the glacial acetic acid had turned blue as CuBr#sub[2] is dissolved bythe acetic acid. The insoluble CuBr was collected by vacuum filtration,and the cake washed three times with -20 °C methanol, then three timeswith -20 °C diethyl ether. The now damp white powder was collected intoa 50 mL RBF and dried under vacuum overnight.
=== PolyGMA synthesis<polygma-synthesis>We synthesized polyGMA according to a previously publishedprotocol.@parkatzidis2023 Briefly, to a 20 mL scintillation vial with amagnetic stirbar, 2.84 g GMA (0.02 mol), 10 μL PMDETA (4.8e#super[-5]mol), 0.001 g CuBr (8e#super[-6] mol), 2.84 mL of DMSO and 70 μL of EBPA(4e#super[-4] mol) is added. The reaction was sealed with a rubberseptum stirred at 60 °C overnight, after which it was diluted with THF,passed through a short plug of alumina, and precipitated into coldmethanol.
=== pGMA_-b-_BMA and pGMA_-co-_BMA synthesis<pgma_-b-_bma-and-pgma_-co-_bma-synthesis>We synthesized linear copolymers by adapting a previously publishedprotocol.@santos2019 Briefly, to a 20 mL scintillation vial 29 µL TMG(2.3e#super[-4] mol), 0.005 g CuBr#sub[2] (2.3e#super[-5] mol), and 1.65g of BMA (0.011 mol) is added. For the block polymer, 1.65 g of pGMA(0.011 mol) and 0.625 mL of DMSO is added. For the random copolymer,1.65 g of GMA (0.011 mol), and 1.25 mL of DMSO is added. The vial iscapped with a rubber septum and nitrogen is bubbled through the solutionfor 15 minutes, after which a magnetic stirbar with an activated copperturning tied around it is added. The reaction is stirred overnight at 30°C, after which it was diluted in THF, passed through a short plug ofalumina, and precipitated into cold methanol. Spectra for all linearpolymers are shown in @fig:linearcopolymer and @fig:linearblock.
#figure(image("Images/C3S1.png", width: 100.0%), placement: auto, caption: [ #super[1]H NMR and GPC of pGMA_-co-_BMA. Spectra recorded at 500 MHz in CDCl#sub[3]. THF used as eluent. ])<fig:linearcopolymer>
#figure(image("Images/C3S2.png", width: 100.0%), placement: auto, caption: [ #super[1]H NMR and GPC of pGMA and pGMA_-b-_BMA. Spectra recorded at 500 MHz in CDCl#sub[3]. THF used as eluent. ])<fig:linearblock>
=== PolyBIEM (pBIEM) synthesis<polybiem-pbiem-synthesis>We synthesized polyBIEM by RAFT polymerization. To a 20 mL scintillationvial 5.56 g of BIEM (0.02 mol), 0.014 g CPDT (0.04 mmol), 0.0013 g AIBN(0.008 mmol), 5.56 mL dioxane, and a new magnetic stir bar is added. Thevial is capped with a new rubber septum and degassed for 15 minutes bysparging with nitrogen using new needles. After degassing, the vial isadded to an aluminum reaction block thermostatted at 60 °C to reactovernight. The next day, the reaction was diluted with dioxane,precipitated into cold diethyl ether, and dried under vacuum at 0.001mbar overnight.
=== Comb polymer synthesis<comb-polymer-synthesis>The synthesis procedure for comb polymers follow similar reaction setup.Reagents and detailed conditions are described after the generalprocedure is outlined and in @combconditions. Synthesis wasundertaken according to an adapted protocol for synthesizingpolyGMA@catalao2015 _via_ SARA ATRP. For each reaction, reagents wereprepared in a 20 mL scintillation vial. The vial was sealed with arubber septum and after reagent dissolution, the vial was degassed bysparging with nitrogen for 15 min, after which either a stir bar withactivated copper turnings wrapped around it or 0.011 g iron powder wasadded to the vial (@combconditions). The vial was added to an aluminum reactionblock at varying temperatures for one hour, until the reaction hadincreased in viscosity. The crude polymer was diluted in THF and passedthrough a short plug of neutral alumina to remove dissolved coppersalts, precipitated in -20 °C methanol, and dried under vacuum at 0.001mbar for one hour. The polymer was used immediately after drying.
PolyBIEM_-g-_(GMA_-co-_BMA): 2.8 g GMA (0.02 mol), 0.064 g pBIEM(4e#super[-4] mol), 0.061 g TPMA (2e#super[-4] mol), 0.0045 gCuBr#sub[2] (2e#super[-5] mol), and 2.8 mL of 70/30 v/v toluene/DMF.
polyBIEM_-g-_(GMA_-co-_BMA): 2.8 g BMA (0.02 mol), 2.8 g GMA (0.02 mol),0.064 g pBIEM (4e#super[-4] mol), 0.061 g TPMA (2e#super[-4] mol),0.0045 g CuBr#sub[2] (2e#super[-5] mol), and 5.6 mL of 70/30 v/vtoluene/DMF.
PolyBIEM_-g-_(GMA_-b-_BMA): 2 mL of BMA (0.014 mol), 1 g of polyBIEM_-g-_GMA(0.007 mol), 0.0045 g CuBr2 (2e#super[-5] mol), 64 µL Me6TREN(2e#super[-4] mol), and 4 mL of 70/30 v/v toluene/DMF.
PolyBIEM_-g-_BMA: 2.8 mL of BMA (0.02 mol), 0.064 g of polyBIEM (0.007mol), 0.0045 g CuBr#sub[2] (2e#super[-5] mol), 64 μL Me6TREN(2e#super[-4] mol), and 2.8 mL of 70/30 v/v toluene/DMF.
PolyBIEM_-g-_(BMA_-b-_GMA): 2 g BMA (0.014 mol), 1 g pBIEM_-g-_BMA (0.007mol), 0.061 g TPMA (2e#super[-4] mol), 0.0045 g CuBr (2e#super[-5] mol),and 2.8 mL of 70/30 v/v toluene/DMF.
#figure( placement: auto, align(center)[#table( columns: 4, align: (center,right,right,right,), table.header([Comb Polymer], [Reducing agent], [Temperature (°C)], [Methanol acidified?],), table.hline(), [PolyBIEM_-g-_GMA], [Iron], [30], [No], [polyBIEM_-g-_(GMA_-b-_BMA)], [Copper], [30], [Yes], [PolyBIEM_-g-_(BMA_-b-_GMA)], [Copper], [50], [Yes], [PolyBIEM_-g-_BMA], [Copper], [50], [Yes], [PolyBIEM_-g-_(GMA_-co-_BMA)], [Iron], [30], [Yes], )] , caption: [Synthetic setup for comb polymerizations.] , kind: table )<combconditions>
#figure( placement: top, align(center)[#table( columns: 4, align: (center,right,right,right,), table.header([Name], [GMA (%)], [Mn (g/mol)], [Đ],), table.hline(), [pGMA], [100], [5,907], [1.12], [pGMA_-b-_BMA], [45], [11,436], [1.12], [pGMA_-co-_BMA], [50], [11,942], [1.09], [pBIEM], [---], [57,641], [1.3], [pBIEM_-g-_GMA], [100], [275,294], [1.17], [pBIEM_-g-_BMA], [---], [415,543], [1.36], [pBIEM_-g-_(GMA_-b-_BMA)], [55], [491,777], [1.24], [pBIEM_-g-_(BMA_-b-_GMA)], [45], [551,826], [1.31], [pBIEM_-g-_(GMA_-co-_BMA)], [51], [423,522], [1.3], )] , caption: [Tabulated properties of polymers used in this study.] , kind: table )<combproperties>
=== Polymer characterization<polymer-characterization>Polymer molecular weight was determined by GPC. GPC was conducted on anAgilent 1260 with a PL gel 5 μm guard column and three 5 μm analyticalmixed C columns (Agilent). THF was used as the eluent at a flow rate of1 mL/min. The column was standardized with pMMA calibration standardsand toluene was used as a flow marker. #super[1]H NMR spectra weremeasured on a Bruker Avance 500 spectrometer with deuterated chloroformas the solvent. Properties are compiled in @combproperties.
=== Sample preparation<sample-preparation>In general, to prepare a crosslinked thermoset, polymer was dissolved inacetone, and succinic acid was added to create a solution with a 1:1molar ratio of oxirane:COOH. 0.05 eq of TBD was added as a catalyst andthe solution was stirred at 100 °C until most acetone had evaporated anda partially crosslinked solid remained. The polymer was then pressed ina mold at 150 °C for 10 minutes, and the flexible solid was cut to shapeand left in an oven at 120 °C overnight to complete the curing process.
=== DMA<dma>To determine the modulus of the crosslinked samples, I performed DMAmeasurements with a DMA 850 (TA Instruments) equipped with a tensionclamp. Frequency sweeps were performed at 0.01 % strain with a preloadof 0.01 N at room temperature.
=== Swelling ratio<swelling-ratio>To determine their equilibrium swelling ratio, fragments of each polymerwere weighed and added into excess xylenes, chosen to be a good solventfor these polymers.@kaya2002 The samples were then placed in an aluminumreaction block thermostatted to 100 °C for 48 hours to reach theirequilibrium swelling ratio. After the 48 hours, the samples werecarefully removed or filtered from the xylene and weighed, with thedifference being used to calculate the swelling ratio.
=== Toughness measurements<toughness-measurements>To determine the toughness of the crosslinked samples, I conducteduniaxial extension measurements with a Texture Analyzer (StableMicrosystems) equipped with a 50 N load cell. The crosshead displacementrate was controlled at a speed of 0.1 mm/s. The samples were stretcheduntil the samples broke at the center of the sample in between thegrips.
== Results and discussion<results-and-discussion>=== Controlled synthesis of reactive combs<controlled-synthesis-of-reactive-combs>We synthesized a suite of different comb polymer architectures with thegoal of determining how graft density and structure affected eventualnetwork modulus and toughness, as well as a set of analogous linearpolymers. I kept monomer ratio and degree of polymerization consistentbetween linear and comb grafts to best compare resultant materialproperties. I synthesized four comb polymers: pBIEM_-g-_GMA (homopolymerGMA side chains), pBIEM_-g-_(GMA_-co-_BMA) (random GMA copolymer sidechains,inset of @fig:combsynthesis b), pBIEM_-g-_(GMA_-b-_BMA) ("hidden" GMAblock copolymer sidechains, inset of @fig:combsynthesis c), andpBIEM_-g-_(BMA_-b-_GMA) ("exposed" GMA block copolymer sidechains, inset of @fig:combsynthesis d). I then synthesized three linear polymersanalogous to the sidechains of the comb polymers: pGMA (compared to combpolymers with homopolymer side chains), pGMA_-co-_BMA (compared to combpolymer with random side chains), and pGMA_-b-_BMA (compared to combpolymers with both "hidden" and "exposed" sidechains).
#figure(image("Images/C3F1.png", width: 100.0%), placement: auto, caption: [ SARA-ATRP produces narrowly dispersed, well defined comb polymers. (a) Illustrations of the monomers used in this paper. (b) GPC chromatogram showing the shifting of precursor polymer to earlier elution times, demonstrating the successful chain extension with GMA_-co-_BMA. (c) GPC chromatogram showing successful chain extensions of pBIEM with first GMA and then BMA to create block copolymer side chains. (d) GPC chromatogram showing successful chain extensions of pBIEM with first BMA and then GMA to create block copolymer side chains. For (b-d), insets depict the idealized structure of the initial and final polymer. ])<fig:combsynthesis>
First, I synthesized linear polymers _via_ ATRP, and determined they hadwell-defined molecular weights and narrow dispersities _via_ GPC (pGMA: Mn\= 5,907 g/mol, Đ = 1.12; pGMA_-co-_BMA: M#sub[n] = 11,942 g/mol, Đ = 1.09;pGMA_-b-_BMA: M#sub[n] = 11,436 g/mol, Đ = 1.12). I then created copolymers ofGMA and BMA with matched lengths, and measured that all polymers hadcomparable amounts of GMA.
In order to produce graft polymers, I used a \"grafting from\" approachto grow GMA and BMA from a pBIEM backbone (@fig:combsynthesis a).To accomplish this, I polymerized an inimer (a dual functional smallmolecule acting as initiator and monomer) of BIEM _via_ RAFT to create alinear polyinitiator $tilde.op$500 monomers long. I chose CPDT as thechain transfer agent as trithiocarbonates have been shown to notparticipate in the necessary ATRP to create the ensuinglinear-block-comb architectures.@kwak2008 Successful synthesis of pBIEMshowed a modest final Đ = 1.3, higher than would be expected for a RAFTpolymerization (@fig:combsynthesis b). This was likely due tosmall amounts of contaminating crosslinking, as indicated by a smallshoulder in the GPC chromatograms at lower elution times. The Đ wassimilar to other reported polymerizations of BIEM.@stals2013
Using two distinct reversible deactivation radical polymerizations,well-defined and narrowly disperse comb polymers were synthesized.First, I created a comb architecture with random side chains (@fig:combsynthesis b). pBIEM_-g-_(GMA_-co-_BMA) was synthesized by initiatingpolymerization from pBIEM using GMA and BMA as comonomers in a 1:1 molarfeeding ratio (pBIEM_-g-_(GMA_-co-_BMA, M#sub[n] = 423,522 g/mol, Đ = 1.3). Thefinal monomer ratio was confirmed _via_ #super[1]H NMR at 51 mol% GMA(@fig:randombrushnmr). I then created a second comb architecturewith a \"hidden\" block architecture. In this structure, the reactiveGMA monomers are closest to the backbone (@fig:combsynthesis c).pBIEM_-g-_(GMA_-b-_BMA) was synthesized in a sequential polymerizationprocess. First, polymerization of GMA from pBIEM yielded pBIEM_-g-_GMA (Mn\= 275,294 g/mol, Đ = 1.17). Next, BMA was polymerized from the ends ofthe grafted GMA of pBIEM_-g-_GMA to create pBIEM_-g-_(GMA_-b-_BMA) (Mn =491,777 g/mol, Đ = 1.24). Final monomer ratio was confirmed _via_ #super[1]H NMRat 51 mol% GMA (@fig:hiddenblocknmr).Finally, I synthesized acomb with an \"exposed\" block architecture, where the reactive GMAmonomers were furthest away from the comb backbone _via_ sequentialpolymerizations of BMA to create pBIEM_-g-_BMA (Mn 415,543 g/mol, Đ 1.36),and then GMA to create pBIEM_-g-_(BMA_-b-_GMA) (Mn 551,826 g/mol, Đ 1.31)(@fig:combsynthesis d). Feed ratio was confirmed _via_ #super[1]H NMR at 51mol% GMA (@fig:exposedblocknmr). All these \"grafting from\"reactions showed no measurable increase in the dispersity of thepolymers relative to the pBIEM backbone.
#figure(image("Images/C3S4.png", width: 100.0%), placement: bottom, caption: [ #super[1]H NMR of pBIEM_-g-_(GMA_-co-_BMA) after successive polymerizations. Spectra recorded at 500 MHz in CDCl#sub[3]. ])<fig:randombrushnmr>
#figure(image("Images/C3S5.png", width: 100.0%), placement: auto, caption: [ #super[1]H NMR of pBIEM_-g-_(GMA_-b-_BMA) after successive polymerizations. Spectra recorded at 500 MHz in CDCl#sub[3]. ])<fig:hiddenblocknmr>
#figure(image("Images/C3S6.png", width: 100.0%), placement: auto, caption: [ #super[1]H NMR of pBIEM_-g-_(BMA_-b-_GMA) after successive polymerizations. Spectra recorded at 500 MHz in CDCl#sub[3]. ])<fig:exposedblocknmr>
=== Crosslinking and storage modulus of reactive comb polymers<crosslinking-and-storage-modulus-of-reactive-comb-polymers>After successfully preparing the reactive comb polymers, determined theeffect of polymer architecture on the bulk properties of theircrosslinked resins. I began by crosslinking the thermosets withsuccinic acid, chosen due to its small molecular weight andfunctionality (@fig:combmodulus a). Small molecular weightcrosslinkers enable high crosslink density, somewhat offsetting thediluting effect of BMA comonomer, and the carboxylic acid functionalityfacilitates step-growth polymerization with the epoxy groups of GMA. A1:1 ratio of acid:epoxy (succinic acid:GMA) was utilized, again toachieve a high crosslink density. Additionally, a catalyst was usedincrease the rate of crosslinking (TBD at 5 mol% relative to epoxy). Alllinear and comb polymers were crosslinked identically.
#figure(image("Images/C3F2.png", width: 100.0%), placement: auto, caption: [ Comb polymers crosslink to form high modulus materials. (a) Schematic of the crosslinking reaction between glycidyl esters and succinic acid, catalyzed by TBD. (b) Storage modulus of block polymers crosslinked with succinic acid, as measured by DMA. (c) Storage modulus of homopolymers crosslinked with succinic acid, as measured by DMA. (d) Storage modulus of random copolymers crosslinked with succinic acid, as measured by DMA. ])<fig:combmodulus>
We first crosslinked and characterized the modulus of the block polymers(@fig:combmodulus b). Successful crosslinking was identified byvisual inspection. Polymers were measured under tensile conditions at 1Hz (6.28 rad/s) frequency, resulting in storage moduli of pGMA_-b-_BMA =584.0 MPa, pBIEM_-g-_(BMA_-b-_GMA) = 410.0 MPa, and = 410.0 MPa. The linearcopolymer had the highest modulus of the three, and shows some frequencydependency, overlapping with the comb block polymers at lowerfrequencies. Strong phase separation of the BMA blocks resulting in adynamic glass transition is likely the cause of the frequency dependentbehavior of pGMA_-b-_BMA. Both comb block polymers had nearly identicalmoduli.
We then prepared crosslinked homopolymer and homopolymer graftthermosets in an identical manner to the block polymers. At 1 Hz, themeasured storage moduli of pGMA = 2,484 MPa and pBIEM_-g-_GMA = 577.1 MPa(@fig:combmodulus c). In this case, the linear homopolymer had afar higher modulus than the comb homopolymer, showing a roughlyfive-fold increase.
Finally, I prepared crosslinked random copolymer thermosets in the samemanner. At 1 Hz, the measured storage moduli of pGMA_-co-_BMA = 881.0 MPaand = 515.0 MPa (@fig:combmodulus d). Here, the linear polymer hasa modulus almost twice as high as the comb polymer.
The highest modulus value belonged to pGMA and pBIEM_-g-_GMA for linearand comb polymers, respectively, is expected, as the overallcrosslinking density of these samples should be the highest. Ascrosslinking was mediated _via_ epoxy groups, the GMA homopolymers havethe greatest degree of epoxy crosslinks without other comonomers todilute it. More surprising is that all comb polymers consistently show alower modulus compared to their linear counterparts. I hypothesizedthat this was due to increased intramolecular crosslinking occurringbetween comb hairs of block copolymer and homopolymer samples. Indensely grafted comb polymers, GMA monomers would be forced very near toGMA on combs within the same polymer, favoring intramolecularcrosslinks. These intramolecular crosslinks would create loop defectsthat do not contribute to the modulus of the final thermoset.
=== Network architecture effects on material properties<network-architecture-effects-on-material-properties>To interrogate the network structure of each crosslinked polymer, weconducted swelling studies. Fragments of all samples were submersed inmixed xylenes at 100 °C for 48 hours to reach equilibrium swelling, andswelling ratio was determined by the difference in mass before and afterswelling. Resultant values for block copolymers were pGMA_-b-_BMA = 1.85,pBIEM_-g-_(BMA_-b-_GMA) = 1.91, and pBIEM_-g-_(GMA_-b-_BMA) 1.2 times. Forhomopolymers, swelling ratios for pGMA = 1.34 times and pBIEM_-g-_GMA =0.85. Random copolymers pGMA_-co-_BMA = 1.95 and pBIEM_-g-_(GMA_-co-_BMA) =1.68 (@fig:combswelling a). All comb polymers, with the exceptionof pBIEM_-g-_(GMA_-co-_BMA) crumbled into small insoluble particles,indicating either that the solid was held together primarily byintermolecular interactions, or that the network topology was uneven,resulting in rapid material failure.@xu2020 Linear polymers retainedtheir original shape (@fig:combmicro). Additionally, all combpolymers, with the exception of pBIEM_-g-_(BMA_-b-_GMA), showed lowerswelling ratios than their linear counterparts.
#figure(image("Images/C3F3.png", width: 100.0%), placement: auto, caption: [ Swelling of thermosetting polymers. (a) Swelling ratios of crosslinked polymers in mixed xylenes. n = 3 (b) Cartoon depiction of comb copolymers with random (left) and block (right) sidechains. ])<fig:combswelling>
#figure(image("Images/C3S3.png", width: 100.0%), placement: auto, caption: [ pBIEM_-g-_(BMA_-b-_GMA), pBIEM_-g-_(GMA_-co-_BMA), and pGMA_-b-_BMA swelling in xylenes. ])<fig:combmicro>
According to Flory,@flory1943 swelling ratio is directly related todegree of polymerization between crosslinks, which in turn is directlyrelated to modulus. According to the swelling data then, each combpolymer should have a higher modulus. To explain this, after comparingthe storage modulus data to the swelling data, I hypothesized that combpolymers contain a high degree of intramolecular crosslinking comparedto linear polymers. The intramolecular crosslinks would form microgelsand small loop defects, which would not contribute to the formation ofelastically active network chains that are able to dissipate stress andincrease the storage modulus (@fig:combswelling b). Additionally,these intramolecular crosslinks contribute to a more inhomogeneousnetwork structure, which reduces the swelling ratio of crosslinkednetworks, as the network is less easily able to expand to accommodateadditional solvent.@cuthbert2021
If these materials do contain a high degree of intramolecularcrosslinks, they may contain fewer intermolecular covalent crosslinks orentanglements, resulting in reduced bulk mechanical properties.Intramolecular crosslinks would form highly crosslinked microgels, butadjacent polymers are loosely connected. Then, in addition todissociating in solvent, I would expect these materials to have muchlower toughness compared to samples that do not contain as many of thesedefects. To examine this, I conducted tensile tests to determine thetoughness of the crosslinked thermosets.
Each sample was cut into rectangles of approximately 6.5 mm in width and1.5 mm in thickness. The samples were pulled until they broke in halfand the toughness calculated from the measured force, elongation atbreak, and sample cross-sectional area. Block polymers, pGMA_-b-_BMA had atoughness of 180.0 kPa, pBIEM_-g-_(BMA_-b-_GMA) had a toughness of 95.60kPa, and pBIEM_-g-_(GMA_-b-_BMA) had a toughness of 93.30 kPa. In contrast,random copolymers, pGMA_-co-_BMA had a toughness of 578.0 kPa, andpBIEM_-g-_(GMA_-co-_BMA) had a toughness of 616.0 kPa(@fig:combtoughness). Homopolymers, pGMA had a toughness of 248.0 kPa,and pBIEM_-g-_GMA had a toughness of 62.30 kPa.
#figure(image("Images/C3F4.png", width: 100.0%), placement: auto, caption: [ Random comb sidechains suppress intermolecular crosslinking and create tougher materials. Tensile toughness of comb and linear polymers. n = 3 ])<fig:combtoughness>
In the case of both block polymers and homopolymers, the comb polymershad lower toughness. These results are somewhat contrary to conventionalblock polymers, as the phase separation between segments of blockpolymers and resultant self-assembly is routinely exploited to createrubbery microdomains capable of dissipating stress and increasing thetoughness of thermosetting resins. Linear block polymers@he2016 and combblock polymers@moon2021 have been demonstrated to improve the toughnessof epoxy resins, and it was expected that comb polymers with a rubberycore would show superior toughness. I attribute this to intramolecularcrosslinking forming loops and microgels that do not create elasticallyactive network chains. Unsurprisingly, the comb homopolymer showed theworst toughness. BMA is known to increase toughness of crosslinkednetworks,@johnson1994@kim1977 and its absence in the homopolymers leadsto increased brittleness. Most surprisingly, the random copolymersshowed the highest toughness, with the comb polymer even achievinghigher toughness compared to the linear polymers. It is worth notingthat the highest measured toughness of pBIEM_-g-_(GMA_-co-_BMA) was 1,088kPa, versus 883.0 kPa for pGMA_-co-_BMA. I hypothesize that the randomlydistributed BMA monomers space apart the reactive GMA monomers, makingit less probable that they will react to form loops with adjacentsidechains. This tensile data shows that I are ultimately able tocreate high modulus materials with toughness comparable to their linearanalogues using highly crosslinked comb polymers.
== Conclusion<conclusion>We synthesized novel thermosetting comb polymers containing bothreactive glycidyl groups and rubbery butyl methacrylate. Block andhomopolymer comb polymers showed low modulus and toughness due tointermolecular crosslinking creating loop defects. Using a randomcopolymerization of reactive and rubbery monomers, I was able tosuppress intermolecular crosslinks and increase the toughness of highlycrosslinked comb polymers. Through this, I demonstrated a method forcreating very high molecular weight thermosetting polymers with hightoughness and modulus. High molecular weight and a tunable density ofepoxy functionality make this material and attractive option for fastsetting coatings and adhesives.