Published in Journal of Applied Polymer Science (2008) vol. 110(3) pp

Transcription

1 Published in Journal of Applied Polymer Siene (2008) vol. 110(3) pp Struture-property relationships in photopolymerizable polymer networks: Effet of omposition on the rosslinked struture and resulting thermomehanial properties of a (meth)arylate-based system Aliia M. Ortega 1, Sott Kasprzak 2, Christopher M. Yakaki 3,4, Julie Diani 5, Alan R. Greenberg 1, and Ken Gall 2,3,4 1 Department of Mehanial Engineering, University of Colorado, Boulder, CO 80309, USA 2 Woodruff Shool of Mehanial Engineering, Georgia Institute of Tehnology, Atlanta, GA 30332, USA 3 Shool of Materials Siene and Engineering, Georgia Institute of Tehnology, Atlanta, GA 30332, USA 4 MedShape Solutions In, Atlanta, GA 30318, USA 5 Laboratoire d Ingénierie des Matériaux, UMR 8006 CNRS, ENSAM Paris, 151 bd de l hôpital, Paris Correspondene to: A. M. Ortega (aliia.ortega@olorado.edu). ABSTRACT: Photo-initiated polymer networks were formed by opolymerization of tert-butyl arylate with di(ethylene glyol) dimetharylate (DEGDMA) or poly(ethylene glyol) dimetharylate (PEGDMA). The degree of rosslinking was systematially varied by modifying the weight fration and moleular weight of the dimetharylate rosslinking agent. An inrease in effetive rosslink density with inreasing rosslinking agent onentrations was onfirmed by dereasing equilibrium swelling ratios (q) and inreasing rubbery moduli (E R ). Glass transition temperatures (T g ) varied from -22 C to 124 C, inreasing with inreasing DEGDMA ontent and dereasing with inreasing PEGDMA ontent. Tensile deformation behavior (at T g ) ranged from an elastomeri-like large-strain response for lightly rosslinked materials to a small-strain brittle response for highly rosslinked networks. At low rosslinking levels, the strain-to-failure of the network polymers dereased quikly with inreasing rosslinking agent onentration. The stress at failure demonstrated a more omplex relationship with rosslinking agent onentration. The effet of omposition on network struture and resulting properties (q, strain-to-failure) dereased as the rosslinking agent onentration inreased. The results reveal trade-offs in T g, E R, strain-to-failure, and failure stress with omposition and network struture, and are disussed in light of the wide range of potential appliations suggested in the literature for (meth)arylate-

2 based photopolymerizable polymer networks inluding biomaterials and shape-memory polymers. Key words: struture-property relations, photopolymerization, rosslinking, mehanial properties, glass transition INTRODUCTION Crosslinking has been shown to greatly influene the physial and mehanial properties of polymers. Crosslinked network polymers demonstrate enhaned reep resistane, improved dimensional stability, inreased thermal stability, inreased mehanial strength, and inreased resistane to solvent absorption 1,2 ompared to unrosslinked linear polymers. Network polymers an be made through the seondary rosslinking of initially linear polymer hains or the single-stage polymerization of multifuntional monomers. Photopolymerization of multifuntional monomers as a means to reate network polymers is of inreasing interest due to the many advantages of photopolymerization suh as spatial and temporal ontrol, reation speed, and the possibility of ex vivo or in situ polymerization for biomedial appliations. 2,3 Photopolymerized networks produed from multifuntional (meth)arylated monomers have been studied for use in a wide range of appliations suh as gas separation membranes, 4 hydrogels, 3,5,6 tissue-engineering, 7 orthopedis, 8 drug delivery, 6 dentistry, 9-11 and shape-memory materials for ardiovasular appliations. 12,13 Suh systems are produed by the opolymerization of mono-(meth)arylate and multifuntional (meth)arylated monomers or the homopolymerization of multifuntional monomers. Due to the omplex nature of the polymerization kinetis and the resulting struture of suh systems, proessing/struture/property relationships are not always straightforward and need to be fully haraterized for optimal use in a wide range of appliations requiring different physial and mehanial properties. For example, photopolymers have been historially used in thin film oating appliations and dentistry, whih both require hard, deformation-resistant polymers ahieved through high rosslink densities. However, emerging biomedial appliations in tissue engineering or shapememory polymers for minimally invasive surgery require soft, deformable polymers with tuned glass transition temperatures (T g ) over a wide range of rosslink densities. Moreover, biomedial

3 appliations demand a more fundamental understanding of mehanial properties beyond elasti modulus, whih is the property typially presented in the photopolymer literature. A number of studies have examined how photopolymerization kinetis and aspets of the resulting struture of (meth)arylate-based systems are influened by monomer properties and reation onditions suh as light intensity, reation temperature, omonomer omposition, monomer hain length between double bonds (monomer size), and monomer funtionality. 9,14-17 These studies have demonstrated that properties suh as the rate of polymerization, pendant double-bond reativity (heterogeneity) during polymerization, and final double-bond onversion of the resulting networks are dependent on various monomer and reation parameters. Additionally, it has been shown that parameters suh as omonomer omposition, monomer size, and monomer funtionality influene the effetive rosslink density of the final network systems. 15,18 Suh hanges in the rosslink density ultimately result in shifts of the physial and mehanial properties of the resulting networks. Studies on the haraterization and quantifiation of the evolution of properties suh as swelling apaity, T g, distribution parameter (a measure of heterogeneity), and elasti modulus with modifiations to omonomer omposition and monomer moleular weight (and inherently network struture) in photopolymerized (meth)arylate systems have been reported. 9,15,18-21 However, less attention has been given to the evolution of deformation behavior and orresponding mehanial properties of suh systems aross a wide ompositional (and strutural) range. Knowledge of suh properties and their link to ompositional and strutural modifiations, as well as more ommonly reported properties suh as T g and elasti modulus, is important due to the varying physial and mehanial requirements of the many possible appliations for suh materials. The purpose of this study is to present a fundamental, systemati evaluation of the oupled struture/property evolution in photopolymerizable (meth)arylate-based networks in terms of their omprehensive mehanial response, inluding stiffness, tensile strength, and deformation limits. Network strutures were produed from the opolymerization of monoarylate and dimetharylate monomers. The onentration, as well as the length, of the dimetharylate monomer was systematially varied with respet to that of the monoarylate monomer as a means to ontrol the resulting rosslink density. The strutural evolution of the materials was haraterized in terms of properties suh as swelling apaity, shifts in T g, and

4 small-strain elasti rubbery modulus (E R ) development. The effet of struture on the resulting tensile deformation behavior to failure (at T g ) of the materials and orresponding mehanial properties suh as strain-to-failure, stress at failure, and tensile toughness was also determined. This work will provide further insight into the property evolution of photopolymerizable polymer networks, as well as highlight and quantify the effet of omposition and resulting struture on a broad range of mehanial properties. This will help to further demonstrate how simple modifiations to polymerization onditions an be used to optimize the physial and mehanial properties of these networks for a wide range of potential appliations. EXPERIMENTAL Materials Network materials were prepared by the opolymerization of tert-butyl arylate (tba) with one of three dimetharylate rosslinking agents: (1) di(ethylene glyol) dimetharylate (DEGDMA), (2) poly(ethylene glyol) dimetharylate, M n =550 (PEGDMA550), or (3) poly(ethylene glyol) dimetharylate, M n =750 (PEGDMA750). Material harateristis are presented in Table 1 and inlude the average number of repeat units (PEG repeat units for the three rosslinking agents) and moleular weights (MW), whih are number-average moleular weights for the PEGDMA materials. The three rosslinking agents have similar bakbone hemistries but differ in hain length (number of PEG repeat units). Comonomer solutions were produed by mixing varying weight perentages of one of the rosslinking agents with tba. The ompositions evaluated in this study are provided in Table 2, where ompositions are listed in terms of weight perent (wt%) and mole perent (mole%) rosslinking agent. The photoinitiator 2,2-dimethoxy-2-phenylaetophenone was added to the omonomer solution at a onentration of 1 wt% of the total omonomer weight and mixed manually until fully dissolved. All materials used in the polymer synthesis were purhased from Aldrih and used as-reeived. Methods Glass slides were oated with Rain-X (SOPUS Produts), whih ats as a non-reating release agent, prior to the addition of the omonomer solution. The omonomer-photoinitiator mixture was injeted between two oated glass slides, whih were separated by 1 mm spaers

5 and seured by binder lips. The entire onfiguration was then exposed to UV light (Blak-Ray, Model B100AP) for 10 minutes (intensity ~8 mw/m 2 ). Near-infrared (NIR) spetrosopy (Niolet 750 Magna FTIR Spetrometer) was used to determine the onversion of a representative set of ompositions (0, 4, 40, and 100 wt% rosslinking agent for all three rosslinking systems) subjeted to the polymerization protool. Spetra were aquired from 16 sans with a 2 wavenumber resolution. Samples were sanned pre- and post-photopolymerization and onversions were based on the =C-H absorption peak loated at approximately 6165 m Perent onversions were alulated by subtrating the ratio of the absorbane peak area after to before polymerization from one 23 and multiplying the final value by one-hundred. All tested ompositions demonstrate onversions > 90%, with the exeption of the 100 wt% DEGDMA omposition that reahes onversion levels of approximately 80%. Swelling experiments (n 2) were ompleted by plaing samples approximately 3 mm x 3 mm x 1 mm in size (ut from bulk slide samples with a preision saw) in ~20 ml of 2-propanol (Fisher Sientifi), a solvent for the tba homopolymer. Samples were polished with grit sandpaper prior to testing and initial weights (W i ) were measured prior to immersion into the solvent. Sample weights were then measured periodially until equilibrium (swollen) weights (W s ) were reahed. The equilibrium weight swelling ratio (q) was alulated from the relationship 24 : W q W s i (1) Samples for dynami mehanial analysis (DMA) were ut from bulk slide samples with a preision saw to final dimensions of approximately 4 mm x 1 mm x 17 mm (n 2). Samples were polished with grit sandpaper prior to testing and the ends were wrapped with aluminum foil to prevent failure at the grips during testing. Testing was performed using a TA Q800 DMA under tensile loading with a dynami strain of 0.1%, a frequeny of 1 Hz, a fore trak of 150% and a heating rate of 5 C/min. The glass transition temperature is defined in this study as the peak of the tan( ) urve obtained from DMA testing, where tan( ) is the ratio of the loss modulus to the storage modulus. 25

6 For strain-to-failure testing, flat dog-bone tensile samples were laser-ut from bulk slide samples. Sample geometry was adapted from dimensions stated in ASTM D , with a gage ross-setional area of 3 mm x 1 mm and a gage length of approximately 8 mm. Samples were polished with grit sandpaper prior to testing to remove edge effets aused by the laserutting. Strain-to-failure tests were performed using an MTS Insight mehanial tester with a 100 N load ell. A thermal hamber (Thermraft, In., model LBO X-J8249_1A) with liquid nitrogen ooling was used to ontrol the isothermal test temperature. Eah omposition was tested at its respetive T g (n 2). One the thermal hamber reahed the set temperature, a 10-minute temperature hold was ompleted prior to sample elongation to ensure temperature equilibrium. Tests were run in displaement ontrol at an extension rate of 1 mm/min. Strain was measured externally by a laser extensometer (MTS LX300). Laser tape was plaed on the extreme ends of the sample gage setion to measure overall gage elongation. Initial tests were onduted to ensure that the tape did not influene the mehanial response (stiffness and strain-to-failure) of the samples. To allow the use of data from runs where the laser data were not usable, an effetive gage length (dependent on grip type and elongation) was determined for eah polymer omposition and used to math strain determined by rosshead displaement to that of the laser extensometer for final determination of strain-to-failure values. The effetive gage length was dependent on material omposition and total elongation beause when rosshead displaement data was ompared to laser extensometer strain data, the softer samples (reahing higher elongation levels) experiened larger deformation ontributions from the grip-to-gage setion (whih inreased as elongation levels inreased), resulting in larger effetive gage lengths. The term rosslink density refers to the number of rosslinked hains per unit volume. The number average moleular weight of the polymer between rosslinks ( M ) is inversely related to the rosslink density. From the theory of rubber elastiity the relationship between modulus and M for an elastomeri network is 26 : 3 RT M (2) E R

7 where is the polymer density, R is the gas onstant, E R is the rubbery modulus, and T is the absolute temperature at whih E R is measured. This relationship loses auray at high rosslink densities due to the loss of a reasonable approximation of a Gaussian hain distribution. As an be seen in Equation 2, M is proportional to the ratio of T over E R or: M T E R (3) This relationship has been used in previous studies to gain an understanding of relative hanges in the network struture of similar polymer systems. 4,18 Due to the wide range of ompositions and thus degrees of rosslinking evaluated in this study, the relationship given in Equation 3 is employed to gain a qualitative understanding of the evolution of network struture with hanges in omposition. The relationship given in Equation 2 is used briefly to present a more quantitative estimate of the atual rosslink spaing in the network polymers. However, the limitation of this relationship at high rosslinking levels is noted and alulated should thus be onsidered as relative approximations. M values RESULTS It is well established that the extent to whih a polymer swells in the presene of a solvent an be related to the degree of rosslinking in the network struture, with the level of equilibrium swelling dereasing with inreasing degrees of rosslinking. 1,27,28 Swelling tests were ompleted as an initial method for determining the struture (extent of rosslinking) of the final polymer networks as a funtion of omposition. The unrosslinked polymer (100 wt% tba) ompletely dissolves in 2-propanol, thus indiating negligible hemial rosslinking in the polymer formed from the pure linear monomer. For the remaining ompositions, the equilibrium swelling ratios in 2-propanol derease as the mole% rosslinking agent inreases (Figure 1). The trends in equilibrium swelling ratios as a funtion of rosslinking agent onentration for the two PEGDMA systems are similar and somewhat overlap. However, the equilibrium swelling ratios of the DEGDMA system are larger than those of the two PEGDMA systems for a given molar rosslinking agent onentration (exept for the 26 mole% DEGDMA omposition).

8 Dynami mehanial testing was used to determine the thermomehanial properties of the network systems (Figure 2). For the DEGDMA system, an inrease in rosslinking agent onentration results in an inrease in the temperature at whih the transition from the glassy state to rubbery state ours (i.e. T g ). Additionally, the breadth of this transition inreases as the onentration of DEGDMA inreases. The materials with the two highest DEGDMA onentrations (85 and 100 wt%) appear to evidene transitions at approximately 140 C. However, these transitions are diffiult to distinguish due to the large breadth and redued height of the orresponding tan( ) peaks. For the two systems with PEGDMA rosslinking agents, an inrease in rosslinking agent onentration results in a derease in the temperature at whih the transition from the glassy state to the rubbery state ours. Furthermore, the derease in T g ours more rapidly for the PEGDMA with the higher moleular weight. The neat tba material (0 wt% rosslinking agent) loses mehanial integrity at temperatures just above its glass transition range. As expeted, the addition of a rosslinking agent results in the presene of a rubbery plateau region that enables the rosslinked polymers to maintain mehanial loading at temperatures above their T g regions. For all three systems studied, an inrease in rosslinking agent onentration results in an inrease in the magnitude of the rubbery modulus plateau. The trends in the modulus-temperature data an be seen in Figure 3, where T g and E R are plotted as a funtion of mole% rosslinking agent. T g values of the DEGDMA system inrease in a near-linear manner with inreasing mole% DEGDMA, with some deviation from linearity at low rosslinking levels (Figure 3a). The T g values of the PEGDMA systems derease monotonially with inreasing mole% rosslinking agent. The rate of derease in T g with inreasing PEGDMA onentration lessens at higher rosslinking onentrations. The rubbery modulus values of all three systems inrease with inreasing mole% rosslinking agent (Figure 3b). No data are inluded for the unrosslinked material due to the lak of a modulus plateau in the rubbery region. For the PEGDMA systems, the rubbery modulus values as a funtion of mole% rosslinking agent are similar at low levels of rosslinking. At higher rosslinking agent onentrations the rubbery modulus of the PEGDMA(550) system surpasses that of the PEGDMA(750) system, and the rubbery moduli of both systems approah asymptoti values as the mole% rosslinking agent inreases further. The E R values of the DEGDMA system for a given mole% rosslinking agent tend to be lower than that of the PEGDMA systems and do not reah a plateau in the tested omposition range. Values of T g and E R for the two highest

9 DEGDMA ompositions (85 and 100 wt%) are not inluded in this analysis due to their lak of a well-defined glassy to rubbery transition. To gain a better understanding of the strutural evolution (in terms of the extent of rosslinking) of the evaluated materials, the ratio given in Equation 3 (T/E R ) was determined as a funtion of omposition. Figure 4 shows T/E R as a funtion of mole% rosslinking agent for the three systems studied. Overall, the T/E R ratio dereases with inreasing rosslinking agent onentration. At small rosslinking agent onentrations, a small hange in omposition results in a large hange in the T/E R ratio. However at higher rosslinking agent onentrations, the moleular weight between rosslinks beomes less sensitive to omposition, with T/E R approahing an asymptoti value as a funtion of omposition. For a given mole% rosslinking agent, the T/E R values for the two PEGDMA systems are similar but are larger for the DEGDMA system until ~26 mole% DEGDMA (40 wt%). Representative tensile stress-strain urves (through frature, indiated by an X) of the three network systems are shown in Figure 5. Eah omposition was tested at its determined T g to ontrol inherent differenes in the stress-strain response due to shifts in the relative T g loations. Sine the 85 and 100 wt% DEGDMA materials did not evidene a well-defined glass transition, they were not inluded in the strain-to-failure testing (where failure is onsidered the point of frature). The unrosslinked material (0 wt% rosslinking agent) reahed the extension limits of the equipment prior to frature. As rosslinking agent onentration inreases, the stress-strain behavior of the material transitions from a lassi elastomeri response to that of a stiff network with a pronouned brittle response. Overall strain-to-failure dereases with inreasing mole% rosslinking agent (Figure 6). The trends of strain-to-failure as a funtion of mole% rosslinking agent for the two PEGDMA systems are similar. However, the strains-tofailure for the DEGDMA system are notieably larger than those of the PEGDMA systems for a given mole% rosslinking agent onentration. For all three systems, strain-to-failure is most sensitive to ompositional hanges at low rosslinking agent onentrations, where small hanges in the mole% rosslinking agent result in relatively large hanges in strain-to-failure. This ompositional sensitivity diminishes as the onentration of rosslinking agent inreases. At low rosslinking agent onentrations all three systems evidene an initial derease and subsequent inrease in the average values of stress at failure (tensile strength) with inreasing onentrations of rosslinking agent (Figure 7). Overall, average stress at failure

10 values for the two PEGDMA systems ontinue to inrease as the PEGDMA onentration inreases. The stress at failure as a funtion of mole% rosslinking agent for the two PEGDMA systems is similar at low rosslinking agent onentration. At higher onentrations of PEGDMA rosslinking agent, the stress at failure of the PEGDMA(550) system beomes larger than that of the PEGDMA(750) system. For the DEGDMA system, at higher levels of rosslinking agent onentration, the stress at failure again deeases. Tensile toughness (or work of frature) values, at T g, were determined from the area under the tensile stress-strain urves (Figure 8). The tensile toughness of the three systems appears to derease with inreasing rosslinking agent onentration at very low rosslinking levels. Beyond that point, there is no lear trend in this property with inreasing rosslinking agent onentration, aside from the inrease in tensile toughness of the PEGDMA(550) at its ompositional limit due to the inreased value of stress at failure of that omposition. DISCUSSION In this study the polymer struture, speifially the extent of rosslinking, was modified by systematially (1) varying the weight fration of a dimetharylate rosslinking agent relative to an arylate base monomer and, (2) varying the size (MW or length) of the rosslinking agent used. This methodology results in a transition in both hemistry and struture from a linear polymer (tba hains) to a three-dimensional network struture (DEGDMA- or PEGDMAbased). This transition in struture should our at the (ompositional) point at whih the rosslinking agent onentration is suffiiently high to statistially favor the interation of rosslinking agent moleules with one another rather than with tba moleules/hains. This ompositional point should theoretially our at a 1:2 ratio of the number fration of dimetharylate moleules to monoarylate monomer moleules, i.e. 33 mole% rosslinking agent (48.6, 73.3, and 80.1 wt% rosslinking agent for DEGDMA, PEGDMA(550), and PEGDMA(750), respetively). However, this point of transition should also depend on speifi polymerization onditions as well as rosslinking agent reativities and effiienies. As a onsequene of this hemial and strutural transition, the struture and property relationships determined in this study reflet both an inrease in rosslink density and a hange in hemistry. The eventual disappearane of the linear (tba) hains in the network as the struture approahes that of the pure DEGDMA or PEGDMA homopolymers in the urrent

11 systems implies that the extent of rosslinking or rosslinking density is ultimately limited by the MW (length) of the rosslinking agents employed and that the rosslink density of the systems should eventually saturate as the rosslinking agent onentration beomes suffiiently high. This behavior is demonstrated in Figure 4 where the T/E R parameters for the PEGDMA systems approah asymptoti values at high rosslinking agent onentrations. This strutural and hemial transition and rosslinking density limitation is most likely the underlying ause of the derease in ompositional sensitivity of a number of the tested physial and mehanial properties of these systems (suh as q, E R, and strain-to-failure) at high rosslinking agent onentrations. The derease and eventual plateau in equilibrium swelling ratio with inreasing mole% rosslinking agent (Figure 1) most likely onfirms the expeted inrease and plateau in rosslink density (derease/plateau in M ) with inreasing rosslinking agent onentration. However, due to the hange in hemistry from a tba-based polymer to a PEGDMA-based polymer as rosslinking agent onentration inreases, the derease in equilibrium swelling ould also be interpreted as being a onsequene of a shift in the hemial ompatibility between the network polymer and the hosen solvent. To hek this possibility, samples of PEGDMA(550) and PEGDMA(750) homopolymers were plaed in de-ionized water as well as in 2-propanol to determine equilibrium swelling ratios in the two different liquids. The equilibrium swelling ratios of these pure PEGDMA polymers were slightly higher in water than in the 2-propanol (average values: q water =1.4, q 2-propanol =1.1) due to the more hydrophili nature of the PEGDMA material. However, these values are lower than the equilibrium swelling ratios of the network materials with low mole% rosslinking agent onentration in 2-propanol. This indiates that while some of the derease in swelling apaity with inreasing DEGDMA/PEGDMA ontent may be due to dereasing solvent-polymer ompatibility, it is also due to an inreasing rosslink density. This is also onfirmed by the inrease in E R with rosslinking agent onentration, as will be disussed. The T g values of the studied network systems are dependent on both the rosslinking agent onentration as well as its MW (Figure 3). This is similar to the trend previously reported for photopolymerized networks systems produed by the opolymerization of n-otyl metharylate with DEGDMA and PEGDMA rosslinking agents. 18 An inrease in DEGDMA, the smallest rosslinker, leads to an inrease in T g whereas an inrease in the longer PEGDMA

12 rosslinkers resulted in a derease in T g, with the longest rosslinker (PEGDMA750) resulting in the lowest T g values. When modifying the rosslink density of a system onsisting of two hemially different monomers by ompositional variations, there are two effets ontributing to the resulting T g values, ommonly referred to as the 'rosslinking effet' and the 'opolymer effet'. 18,29-31 The 'rosslinking effet' is due to the addition of juntions between hains in the struture, whih will affet the free volume 32 and at to restrit moleular motion of the polymer hains. 30 The 'opolymer effet' is due to the hange in hemistry that ours in the polymer as hemially different rosslinks are added to the base polymer/monomer. The 'rosslinking effet' will at to inrease T g while the opolymer effet an either inrease or derease T g depending on the hemial and physial nature of the ombined materials, in partiular the inherent onformational rigidity of the bakbones and the intermoleular interations between the various hains. The DEGDMA rosslinking agent appears to at as more of a traditional rosslinker in terms of its effet on T g. This is most likely due to the shorter rosslinks ating to derease the free volume of the system as it pulls the main tba hains together. This would effetively inrease the temperature at whih larger-sale moleular motions an our. Additionally, the DEGDMA rosslinker does not introdue an appreiable fration of the soft (low T g ) PEG units that are not bound-up lose to a network juntion. The longer PEGDMA rosslinkers result in a opolymer effet, lowering T g by inorporating low-t g moleules into the polymer network. The longer rosslinks might also inrease the free volume of the system and thus derease the temperature at whih moleular motion an our. Thus the redution in T g due to the inorporation of low T g PEG segments inreases with the MW of this segment. However, the restrition of rosslinking on the networks dereases with inreasing PEG MW, also ating to derease T g with inreasing PEG MW. This redution in the restrition of rosslinking with inreasing PEG MW is also refleted in the mehanial properties of this system, with E R and stress at failure dereasing with inreasing PEG MW as the ompositional limits are approahed. Previous studies have evaluated the magnitude of the rosslinking and opolymer ontributions to T g in a number of rosslinked systems however no attempt was made to expliitly separate these effets in the present study. While the longer PEGDMA rosslinkers result in a 'opolymer effet' with respet to T g, the introdution of any of the three rosslinking agents still results in additional (permanent)

13 juntions between hains that restrit moleular motion due to an applied load. This is evidened by the inrease in E R with inreasing rosslinking agent onentration for all three systems, onfirming an inrease in the degree of rosslinking. However, at low rosslinking agent onentrations, E R for the two PEGDMA systems is dominated by the onentration of the rosslinking agent, and is less affeted by the MW of the rosslinker. At higher rosslinking agent onentrations E R is dominated by the MW of the rosslinking agent and is less sensitive to hanges in the rosslinking agent onentration in that range. This results beause at the high PEGDMA onentrations, the struture approahes that of the PEGDMA homopolymer and the rosslink density is limited by the MW of the PEGDMA rosslinker rather than by the added fration of the agent. The shorter PEGDMA(550) hains produe a tighter network than that of the PEGDMA(750) system, resulting in more rosslinking restritions and thus higher modulus values at the ompositional limits. The DEGDMA rosslinking agent should provide the highest rosslink density of the three rosslinking agents sine this monomer has the smallest hain length. However, modulus values of the DEGDMA system are smaller than those of the PEGDMA systems, indiating a lower effetive rosslink density in this system (as refleted in larger alulated T/E R values shown in Figure 4). A similar response was previously reported by Kannurpatti et al. 18 and attributed to the shorter DEGDMA moleules being more prone to yle formation during polymerization than the larger PEGDMA moleules. This tendeny has also been predited by simulation studies done on free radial polymerizations of multifuntional monomers. 14,33 The formation of primary yles during polymerization (where a propagating radial on the main hain reats with a pendant double bond from the same hain) results in fewer effetive rosslinks for a given molar rosslinking agent onentration than a monomer that does not form yles. The less effetive nature of DEGDMA rosslinking is also evident in its larger equilibrium swelling ratios (Figure 1) and larger strains-to-failure (Figure 6) than the PEGDMA systems for the same molar rosslinking agent onentration. A trend towards the theoretial rosslinking limit (based on rosslinking agent MW) is however notied as the DEGDMA onentration approahes 26 mole%, where the properties of the DEGDMA-based system appear to transition from a material that is more lightly rosslinked than the PEGDMA-based materials (resulting in higher swelling and T/E R ratios and strain-tofailure values as well as lower E R values) to a material that is more tightly rosslinked than the

14 PEGDMA-based materials (resulting in lower swelling and T/E R ratios and strain-to-failure values as well as a higher E R values). Strain-to-failure has been shown to derease with inreasing onentration of rosslinking agent in rubbers. 26,34 The same trend is demonstrated in the urrent study, with strain-to-failure being most signifiantly affeted by the rosslinking agent onentration. This property is very sensitive to ompositional hanges at low rosslinking agent onentrations (up to ~10 mole%), whereby a small addition of rosslinking agent results in a signifiant derease in the strain-to-failure, similar to trends reported for rubber. 26 Thus, small additions of permanent rosslinks hinder moleular motion enough to impede large-sale deformation. The general trend in strain-to-failure and T/E R ( M ) as a funtion of molar onentration of rosslinking agent appear very similar, suggesting a orrelation between these two properties. orrelation is observed more diretly in Figure 9 in whih strain-to-failure is plotted as a funtion of the T/E R ratio. The data demonstrate that there is a near linear relationship between strain-tofailure and This M, whih has also been reported for peroxide-vulanized rubbers (from work of Morell and Stern, in Treloar 35 ). At low T/E R values (high rosslink densities) the strain-tofailure values are similar for the three rosslinkers. However, at larger T/E R values (low rosslink densities) the responses of the three rosslinking systems deviate, with the PEGDMA(550) system resulting in the largest strains-to-failure and the DEGDMA system resulting in the smallest. Sine this orrelation onsiders rosslink density, as measured through rubbery modulus, rather than rosslinking agent onentration, it annot be simply explained based on the effetiveness of the rosslinkers. The trend in strain-to-failure with rosslinker moleular weight at low rosslink densities is not fully understood; however, relative heterogeneity of the resulting network systems may be one fator ontributing to this trend. A redution in strain-to-failure with inreased rosslink density has also been reported for a number of network systems (beyond traditional rubbers and elastomers) with varying rosslink densities produed by modifying the rosslinking agent onentration prior to opolymerization, the rosslinker moleule size, or the pre-polymerization solvent onentration However, in ontrast to suh previous studies where mehanial testing was performed at room temperature, in the urrent study eah polymer is tested at the determined T g to ensure that eah material is in a similar state suh that the reported trends were mainly a result of the effetive rosslink density in eah system and not of relative shifts in T g whih an also have a first-order effet on

15 deformation and strain-to-failure.. For example, if testing on the urrent system was done at room-temperature, strain-to-failure ould inrease with addition of a PEGDMA rosslinker simply beause the T g is shifted towards the test temperature, providing a visoelasti (not glassy) material for whih failure strains would be naturally higher. Ensuring that eah material is tested while in similar states allows a more diret orrelation between the struture and resulting deformation behavior as well as determined mehanial properties, whih is the main purpose of this study. Related work on strain-to-failure as a funtion of temperature in the glassy, visoelasti, and rubbery regimes of photopolymerized polymer networks was reently reported. 40 The average stress at failure demonstrates a onsistent initial derease with inreasing rosslinking agent onentration. This is also seen in Figure 5 by the loss of the upturn in the stress-strain response at large elongations as rosslinking agent onentration inreases. This upturn in stress, ommonly seen in rubber-like or elastomeri materials, has been attributed to limited hain extensibility and/or strain-indued rystallization. 1,41 The stress at failure then inreases again as the rosslink onentration inreases and a more highly rosslinked struture forms. Failure stress is most affeted by the mole% rosslinking agent at low rosslink agent onentrations, but then beomes additionally dependent on the MW of the rosslinking agent as the struture transitions to the PEGDMA-based network and again the inrease in rosslink restritions with dereasing PEG MW results in the shorter PEG system demonstrating higher stress at failure values at the ompositional limits. The DEGDMA system evidenes a seond drop in failure stress at high rosslinking agent onentrations. This ould be due to a more heterogeneous struture in the high DEGDMA material, whih has been suggested as a ause of dereased ultimate properties in network systems due to a non-uniform stress distribution ourring in the network and weak interfaes between highly rosslinked (mirogel) regions. 26,38 Earlier studies (simulation and experimental) on networks formed from multifuntional monomers have onluded that short rosslinkers, suh as DEGDMA, result in more heterogeneous strutures than longer rosslinkers 14,18 and that heterogeneity inreases with inreasing rosslinking agent onentration. 14,15,18 An inreased heterogeneity may also be indiated in the present system by the broader glass transition region of the 40 wt% DEGDMA system (ompared to the high PEGDMA onentration systems), whih has been suggested to

16 tested. 26 Tensile toughness (as alulated from the area under the stress-strain urves) appears to orrespond to a wider struture. 30,42 M distribution and/or a general heterogeneity in the network When the stress at failure is analyzed as a funtion of T/E R, a different trend is observed (Figure 10) whereby the stress at failure dereases rapidly as the distane between rosslinks is inreased, and then plateaus or inreases somewhat with a further inrease in T/E R. A number of different trends have been reported for the effet of rosslink density on the ultimate strength (or stress at failure) of rubber-like and elastomeri materials. 35,41,43,44 One ommonly reported trend for rubberlike materials is an inrease in strength in the early stages of rosslinking followed by a redution in strength as rosslinking is inreased, 34,35,45 typially attributed in part to the extent of hain orientation at high elongations, as a funtion of the degree of rosslinking. A more linear relationship between stress at rupture and rosslink density has been observed for polydimethylsiloxane (PDMS) networks, 43 whih were reported as being resistant to strainindued rystallization. inreasing In ontrast, an exponential-like derease in stress at failure with M for PDMS networks has also been reported. 41,44 It has been suggested that the various trends reported ould result from basi strutural differenes between the polymers be relatively unaffeted by the onentration of rosslinking agent for the ompositional ranges evaluated (Figure 8). The PEGDMA(550) system appears to experiene a slight inrease in toughness as the omposition approahes that of the PEGDMA(550) homopolymer due to the inrease in stress at failure at high PEGDMA onentration. The highest toughness values are observed in the most lightly rosslinked materials, due to their large elongations. If ompositions with even smaller onentrations of rosslinking agent were studied, it is possible that a further inrease in toughness would result due to an assumed inrease in strain-to-failure and possible further inrease in stress at failure (Figures 6 and 7). Otherwise, additional means need to be employed (suh as hanges in deformation temperature and/or rate 40 or the hemial nature of the (meth)arylates) to allow for tailorability of toughness. Impliations The rosslinked tba system is highly tailorable with respet to T g, E R, and strain-tofailure by simple modifiations in rosslinking agent onentration and MW. The average T g of

17 these systems ranged from -22 to 124 C; average E R ranges from 0.4 to 70 MPa and average strain-to-failure ranges from < 10% to 357%. Therefore, this system should enable property optimization for a number of appliations. However, there are limitations on the degree of optimization for some properties. For example, there is a signifiant trade-off between E R and strain-to-failure, so a stiff material with a large strain-to-failure would be diffiult to obtain. This is demonstrated in Figure 11 where strain-to-failure is plotted as a funtion of E R as well as (alulated from the relationship given in Equation 2). Due to the dependeny of the alulated value of M on T (whih is different for eah omposition), a fit between E R and the alulated M was determined for the ombined data of all three systems. This fit was used to determine M values of M (for a given E R ) listed on the top x-axis in this figure. Deviation of the fit from alulated values of M from speifi data points inreases as E R inreases, reahing approximately a 25% differene for the most highly rosslinked ompositions of eah of the three systems. Figure 11 additionally demonstrates the diminishing sensitivity of strain-to-failure to E R as E R inreases (and M dereases). The strain-to-failure dereases as E R begins to inrease (and M dereases), suh that at low rosslinking levels a small inrease in E R generates a substantial derease in strain-to-failure. As E R ontinues to inrease, strain-to-failure ontinues to derease, however at a muh smaller rate, with the PEGDMA(550) system reahing an apparent asymptoti value. Changes in monomer hemistry would need to be explored further to determine how additional properties suh as strain-to-failure, toughness, and E R ould possibly be modified and further optimized. Nonetheless, this study quantifies the inherent trade-offs between major physial and mehanial properties of (meth)arylate-based photopolymerizable systems. The effet of rosslinker onentration and MW on a number of the studied properties trade-off as the rosslinking onentration is inreased. For example, at low rosslinking onentrations, a mixture of rosslinker of varying MW ould be used to obtain a desired T g, with E R, q, and strain-to-failure being somewhat unaffeted as long as the required rosslink onentration is maintained. However, additional drawbaks and benefits of the length of the rosslinking agent must be onsidered. For example, the longer rosslinkers ould be onsidered

18 superior to the shorter DEGDMA rosslinker in that they form a more homogeneous system, are less prone to form yles during polymerization, and thus provide more effetive rosslinking, as indiated via higher E R values and larger ahievable tensile strengths. Conversely, the possible values of T g from systems using the longer PEGDMA rosslinks are all low and the shorter rosslinking agent (DEGDMA) would be needed if a higher T g were required. Additionally, the primary yles that form during the polymerization of the DEGMDA system result in larger strains-to-failure for a given molar onentration of the rosslinking agent, whih ould be a desired harateristi. The results have onsiderable impat on the use of suh networks for shape-memory behavior where, similar to elastomers, materials with varying and tailorable strain-to-failure and rubbery modulus values are highly desired. Similarly, the development of photorosslinked hydrogels for tissue-engineering appliations will benefit from the results due to varying requirements of stiffness and extensibility. In shape-memory polymers, low rosslink densities result in large reoverable strains but low fore generation under external onstraint, while high rosslink densities result in small reoverable strain levels but large reoverable fores under external onstraint. Additionally, it is neessary to be able to tailor the reovery temperature and/or time of shape-memory polymers for speifi appliations, whih an be aomplished by adjustment of T g. CONCLUSIONS The studied systems are highly tailorable with respet to T g (-22 C to 124 C), E R (0.4 to 70 MPa), and strain-to-failure at T g (< 10% to 357%) by simple modifiation of the fration and MW (length) of the rosslinking agent added to the monoarylate base monomer. T g is dependent on both the MW of the rosslinking agent and its onentration in the omonomer mixture. For the PEGDMA systems, at low onentrations of rosslinking agent, E R is mainly driven by the rosslinking agent onentration. At higher rosslinking agent onentrations E R is more dependent on the MW of the rosslinking agent. The same dependene on rosslinking agent onentration and MW are observed for the stress at failure of these network systems. Strain-to-failure is more dependent on rosslinking agent onentration, as opposed to rosslinking agent MW. There is a signifiant trade-off between E R and strain-tofailure for the three systems in the study.

19 DEGDMA is a less effetive rosslinking agent than the longer PEGDMA moleules. This is most likely due to the greater propensity of the DEGDMA moleule for forming yles during polymerization. This led to unexpeted trends in mehanial properties of the DEGDMA system, suh as higher swelling apaities, lower E R values, and higher strains-to-failure ompared to those of the PEGDMA systems for a given molar onentration of rosslinking agent. A more heterogeneous struture in materials with a high DEGDMA onentration is a possible ause of apparently premature tensile failure, resulting in a redued value of stress at failure. Due to the use of ompositional hanges between a monoarylate monomer and a dimetharylate rosslinking agent as a means to ontrol rosslinking density, the majority of the physial and mehanial properties of this system demonstrate a redued sensitivity to ompositional hanges at higher rosslinking agent onentrations, sine at this point the network system primarily onsists of the rosslinking agent. A different trend in suh properties will most likely our if the rosslink density were to be ontrolled by other means suh as the rosslinking of pre-existing linear polymer hains. With the wide range of physial and mehanial properties ahievable in photopolymerizable systems suh as those used in this study, a signifiant degree of property optimization an be ahieved for a wide range of appliations. This study demonstrates the ease with whih a number of key physial and mehanial properties of photopolymerized network systems an be tailored by simple ompositional modifiations. Additionally, the present work quantifies the trade-offs in the mehanial properties of suh systems. Aknowledgments This projet was supported by Grant Number F31AR from the National Institute Of Arthritis And Musuloskeletal And Skin Diseases (to A.M.O.). The ontent is solely the responsibility of the authors and does not neessarily represent the offiial views of the National Institute of Arthritis and Musuloskeletal And Skin Diseases or the National Institutes of Health. Dr. Diani is grateful to the CNRS for its finanial support: CNRS-Etats-Unis 2006 #3499. The authors gratefully thank Dr. Chris Bowman and Dr. Neil Cramer for their assistane with NIR testing.

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