Oligomer for UV-Curing

Photogeneration of gelatinous networks from pre-existing polymers

Photogeneration of Gelatinous Networks from Pre-Existing Polymers

Gregory T. Carroll,1 L. Devon Triplett,1 Alberto Moscatelli,1 Jeffrey T. Koberstein,2Nicholas J. Turro1,2

1Department of Chemistry, Columbia University, MC 3157, New York, New York 100272Department of Chemical Engineering, Columbia University, New York, New York 10027

Received 14 November 2010; accepted 23 December 2010

DOI 10.1002/app.34133
Published online 20 April 2011 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: In this manuscript we report the crosslinking of pre-existing macromolecules in solution through the use of photoactive benzophenone chromophores. We show that a bifunctional crosslinker composed of two benzophenone chromophores as well as a single benzophenone chromo- phore crosslink poly (butadiene) and poly (ethylene oxide) in solution to form insoluble gels when irradiated with UV light. The molecular weight between crosslinks of the photo- generated gels was compared for the two crosslinkers, for an equivalent amount of benzophenone chromophores in each solution, by measuring the swelling ratio of the gels formed.

INTRODUCTION

The physical properties of polymer solutions can be modified by crosslinking the chains to form an extended network (Fig. 1). Such networks offer increased durability1 compared to their uncros- slinked counter-parts and are pertinent in a variety of current and future applications including absorbent2,3 and elastic4 materials, photoresists for micro- and nanofabrication,5,6 membranes,7,8 tissue engineering,9 and controlled drug release.10 The properties of crosslinked polymers depend on the crosslink density. As more crosslinks are formed the viscosity of the sample will increase. At the gel point the system undergoes a sharp transition at which the network becomes macroscopic and acts like an elastic solid rather than a viscous liquid.11 Such a network is said to be infinite with every chain form- ing a bond with at least two other chains. When such a gel is placed in a good solvent, rather than dissolve, the gel will absorb the liquid and swell. Although a gel has mechanical properties associated with solids such as a nonzero sheer modulus, a large fraction of the gel can consist of solvent.12 Some gels

Correspondence to: N. J. Turro (njt3@columbia.edu).

Contract grant sponsor: National Science Foundation; contract grant numbers: 0717518, DMR 0703054

Journal of Applied Polymer Science, Vol. 122, 168–174 (2011)VC 2011 Wiley Periodicals, Inc.

Gels formed from the bifunctional benzophenone cross- linker were shown to contain more than twice as many crosslinks compared to gels formed from the crosslinker composed of a single benzophenone chromophore. EPR measurements of a nitroxide derivative absorbed into the gels further supported a higher crosslink density for the gels formed from the bifunctional benzophenone crosslinker.VC 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 168–174, 2011

Key words: crosslink; gels; benzophenone; photochemistry; swelling

can absorb an amount of solvent that is hundreds of times the mass of the dry gel.2,3

A variety of photochemical methods have been reported for crosslinking polymers.6 Many methods rely on crosslinkable pendant groups in the chain. Polymers containing groups susceptible to polymer- ization via a photo-initiated process provide suitable pendant groups for photo-crosslinking. For example, acetylene containing polymers have been crosslinked by irradiation in the presence of tungsten hexacar- bonyl.13 Other methods require only a photo-active functional group. Polymers containing various C1⁄4C groups can be dimerized via a photo-induced cyclo- addition. Specific examples include polymers con- taining pendant cinnamate14 and anthracene15,16groups. Irradiation of polymers containing phenolic OAH groups results in the cleavage of the OAH bond.17 The resulting phenoxyl radicals can dimerize to form a crosslink. The reaction is facilitated in the presence of oxygen, which can abstract a hydrogen atom from the phenolic OAH group. Poly (styrene) (PS) similarly undergoes crosslinking by direct exci- tation of the phenyl ring.18,19 Energy transfer results in the cleavage of a benzylic CAH bond. The result- ing radicals undergo secondary processes that result in crosslinks. Polymers containing photo-active pendant groups that can generate species that react with CAH and other common functionalities upon absorption of a photon have also been used to cross- link polymers. Pendant benzophenone20 and azide21

PHOTOGENERATION OF GELATINOUS NETWORKS 169

Figure 1 Schematic of the transformation of a group of polymer chains (left) into a single gel of crosslinked macromole- cules (right).

moieties have been used to effect crosslinks in this way. Incorporating chromophores that react with CAH and other functionalities into bifunctional mol- ecules provides a more general protocol for cross- linking, allowing networks to be formed from pre- existing polymers that do not contain photo-active groups.22,23 Crosslinks are generated by recombina- tion of macroradicals or recombination of both sides of the bi-functional molecule with a polymer. Mono- functional reactive compounds that can abstract only one hydrogen atom to form radicals are also capable of generating crosslinks in polymers by forming macroradicals that recombine to form covalent bonds.24,25 Even more general, irradiating polymers with deep UV can cause crosslinking by chain scis- sion followed by recombination of the resulting radi- cals.6 This can potentially damage functional groups in the chain, compromising the functional properties of the resulting crosslinked network.

The ability to crosslink polymers without prior chemical derivatization or synthesis of a polymer with pendant photo-active groups is of interest because it avoids the often-laborious and expensive task of synthetic chemistry. We previously demon- strated a photochemical method for crosslinking and patterning pre-existing poly (styrene) (PS) films through the use of bisbenzophenone additives.23 In this report we utilize the methodology for crosslink- ing polymers in solution to form macroscopic net- works. We provide evidence that crosslinking results from the photo-generation of radical species. We compare the swelling properties of networks formed by both bis- and mono-functional benzophenone.

METHODS

bis-BP was synthesized according to a previously reported procedure.23 After chromatography the product was further purified by recystallization from methanol.

Photochemical crosslinking in solution

Both PBD and PEO gels were prepared in solutions that were deoxygenated by bubbling with argon for 10 min followed by irradiation with a Rayonet pho- tochemical reactor containing bulbs that emit at 350 nm. In a typical procedure for preparing PBD gels 3.5 lmol (0.01744 g) of bis-BP were added to a solu- tion of 27.4 lmol (0.01312 g) of PBD in 2 mL of ben- zene. The sample was irradiated for 2 h. In a typical procedure for preparing PEO gels 29.2 lmol (0.01397 g) of bis-BP was added to a solution of 0.097 lmol (0.0194 g) of PEO (200,000 K) (Acros) in 2 mL of ace- tonitrile. PEO gels made with BP (Acros) were simi- larly prepared using a 2 molar amount of BP to keep the amount of chromophores equal between the two samples. Samples were irradiated for 5 h and 40 min.

Swelling measurements

Photo-generated gels were soaked in good solvent for 3 days, changing the solvent each day before per- forming swelling measurements. Gels were then dried at room temperature in a vacuum oven until an unchanging dry weight was obtained. Gels were then soaked in a given solvent for 1 day. Gels were removed from the solvent, lightly tapped with Tef- lon tape to remove excess solvent at the surface and weighed. Gels were then placed back in the vacuum oven to repeat the measurements.

Instrumental measurements

EPR measurements were performed using a Bruker EMX EPR spectrometer. Photo-generated PEO gels ($ 4 h of irradiation with set-up described above) were soaked overnight in a solution of 33.4 lmol (0.00568 g) of 4-Oxo-Tempo in 6.7 mL of acetonitrile. The nitroxide solution was removed, the samples were each rinsed twice with DI water, and EPR spectra were taken of the two gels.

RESULTS AND DISCUSSION

Benzophenone chromophores26 are known to undergo hydrogen abstraction reactions with hydro- gen atom donors such as CAH groups, and thus are

Journal of Applied Polymer Science DOI 10.1002/app 170

CARROLL ET AL.

Figure 2

Structure of photo-active crosslinkers.

because this allows for more than one plausible mechanism for crosslinks to form as shown in Figure 3. Irradiation is expected to produce an excited n–p* state that intersystem crosses to the tri- plet.26 One of several deactivation pathways includes hydrogen abstraction of a nearby CAH group on a polymer chain. Hydrogen abstraction will form radicals that can recombine to form cova- lent bonds. Two potential recombination pathways can result in crosslinks. First, radical centers on the polymer chains can recombine with each other. This requires that the photo-generated macroradicals are located sufficiently close to each other. Note that this pathway may be limited in environments where chain motion is hindered. Second, the inclusion of two benzophenone chromophores supplies an addi- tional crosslinking pathway that circumvents the need to have two macroradical centers in close proximity. Recombination of two benzophenone ketyl radicals at the ends of a single molecule of bis-BP with macroradicals results in crosslinks without the need for two interacting polymer radicals. The main possible side reactions include recombination of the ketyl radicals to produce pinnacol derivatives that may or may not act as extended crosslink bridges and disproportionation products.

capable of crosslinking a wide variety of different polymer types as long as they possess CAH bonds. We examined the solution-phase crosslinking poten- tial of both bis- (bis-BP) and mono-functional benzo- phenone (BP), both of which are shown in Figure 2. Benzophenone is well suited for our studies for a va- riety of reasons. It is generally not reactive when kept in the dark. It can be excited at wavelengths above 350 nm, which is generally less destructive than shorter wavelength UV. Its photophysical and photochemical properties have been well studied, including its propensity for hydrogen abstraction.26

BP has previously been shown to crosslink poly- mers.24 The proposed mechanism involves the formation of macroradicals by hydrogen abstraction followed by recombination. We reasoned that incor- porating multiple benzophenone chromophores into one molecule should increase the chances for cross- linking a variety of polymer types with different CAH bond strengths and in different environments

Figure 3 Two pathways are proposed for crosslinking of polymers by bis-BP. In the first, radicals created on polymer chains by hydrogen abstraction can recombine to form crosslinks. In the second, after bis-BP abstracts hydrogen atoms from polymer chains, recombination between a polymer radical and a benzophenone ketyl radical result in a covalent bond. When this happens on both benzophenone chromophores, a crosslink will result.

Journal of Applied Polymer Science DOI 10.1002/app

PHOTOGENERATION OF GELATINOUS NETWORKS 171

Figure 4 Photo-generated PBD and PEO gels using bis-BP. Gels were also obtained when BP was used as the crosslinker.

To test the versatility of the photo-crosslinking reaction in solution we selected poly (butadiene) (PBD) and poly (ethylene oxide) (PEO), two poly- mers containing different physical and chemical properties. For example, PEO is hydrophilic and expected to result in a hydrogel when crosslinked, whereas, PBD is hydrophobic and is not expected to absorb an appreciable amount of water. Regard- less of the physical and chemical properties, irradi- ation of both polymers in the presence of BP orbis-BP resulted in the precipitation of insoluble material from deoxygenated solutions. Gels photo- generated from bis-BP are displayed in Figure 4. Irradiation of deoxygenated polymer solutions without crosslinker did not result in the formation of gels.

A variety of methods have been used to charac- terize crosslinked networks including small angle

neutron scattering,27 pulsed-field gradient,28,29 and solid-state NMR,30,31 IR,32 AFM,33,34 and rheology35measurements. Swelling measurements provide a convenient technique for characterizing crosslinks in a variety of polymers that allows for the crosslink density to be calculated based on the ratio of the volume of the swollen gel to the dry gel.11,36,37 We compared the crosslink density of PEO gels photo- generated using BP and bis-BP by analyzing the swelling ratios of the two gels using both toluene and acetonitrile as swelling solvents. These solvents were selected because their solubility parameters, 18.2 and 24.3 (MPa)1/2, are slightly above and below that of PEO, 20.2 6 2 (MPa)1/2.38 In addition, the volatility of these solvents was low enough such that a stable weight of swollen gel could be obtained. The PEO gels clearly swell in comparison with the dry gel as shown in Figure 5. From the swelling

Journal of Applied Polymer Science DOI 10.1002/app

172 CARROLL ET AL.

Figure 5 Dry and swollen PEO gels photo-generated by irradiation of acetonitrile solutions containing PEO and BP or bis-BP. Gels were soaked in acetonitrile for 12 h. Similar results were obtained using toluene.

ratio the molecular weight between crosslinks, Mc, can be calculated from the following equation:

1=ðmMcÞ 1⁄4 ðlnð1  m2Þ þ m2 þ v1m2Þ=ð/ðm1=3  m2=2ÞÞ22

(1)

where m is the specific volume of the polymer, m2 is the volume fraction of polymer, v is the Flory inter- action parameter and / is the molar volume of solvent. m2 can be calculated from the swelling ratio,q, according to eq. (2):

q 1⁄4 V=Vo 1⁄4 1=m2 (2)

where V is the volume of the wet gel and Vo is the volume of the dry gel. Mc values and the crosslink density, mx, for BP and bis-BP PEO gels are shown in Table I. The crosslink density was calculated from eq. (3):

mx 1⁄4 1=ðmMcÞ (3)

A higher crosslink density was calculated for thebis-BP PEO gels regardless of the swelling solvent. We conclude therefore that the bis-BP gel is more heavily crosslinked than the BP gel. This is expected since bis-BP has more than one mode of forming crosslinks. When a BP ketyl radical recombines with a

polymer radical, a potential polymer–polymer cross- link is lost. However, when a bis-BP ketyl radical recombines with a polymer radical a polymer–poly- mer crosslink can still be formed if the second chro- mophore recombines either with a polymer radical or another bis-BP ketyl radical attached to a polymer.

We also examined the crosslink density using EPR spectroscopy. The line shape of an EPR spectrum relates to the rotational correlation time of the para- magnetic species.39 Hindered molecules rotate more slowly, making it more difficult to align its magnetic moment with the applied magnetic field. Such a spectrum will typically have unequal peak heights and broaden in comparison to a sample in an envi- ronment where rotation is less hindered. Nitroxide molecules are common EPR probes because they contain stable radicals. We reasoned that if the

TABLE I
Comparison of the Molecular Weight Between Crosslinks (Mc) and Cross-Link Density (mx) for PEO Gels Photo-Crosslinked by BP and bis-BP

Acetonitrile Toluene Crosslinker Mc mx Mc mx

BP 890061500 150620 700064000 220690bis-BP 3600 6 800 380 6 80 1300 6 200 1000 6 150

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PHOTOGENERATION OF GELATINOUS NETWORKS

173

EPR spectra of 4-oxo-tempo in photo-generated gels from (a) BP and (b) bis-BP.

bis-BP gels are more heavily crosslinked than the BPgel, nitroxides absorbed into the gel should give a signal indicative of slower rotation. We incubatedBP and bis-BP PEO gels overnight in acetonitrile sol- utions containing 4-Oxo-Tempo. After removing the nitroxide supernatant we took EPR spectra of the gels (Fig. 6). Each gel showed the characteristic three line EPR spectrum exhibited by nitroxide radicals, indicating that nitroxide absorbed into the gel. The gel photo-generated from BP [Fig. 6(a)] shows a much more intense signal, indicating that nitroxide more easily penetrated this gel. The spectrum of the gel photo-generated from bis-BP [Fig. 6(b)] shows an attenuated signal for the peak at the highest field, which is a sign of a longer rotational correlation time. The peaks for the BP gel are broader; however this can be a result of oxygen penetrating the net- work. We attribute these results to a higher crosslink density in the bis-BP gels since a higher crosslink density is expected to make the gel less permeable and to inhibit the rotational diffusion of molecules that are absorbed into the gel. In conjunction with the swelling studies described above, we conclude that bis-BP is a more efficient crosslinker than BP.

CONCLUSIONS
We have shown that both BP and bis-BP are capable

of crosslinking polymers in solution. In comparison

with BP, analysis of the swelling ratios of photo- generated PEO gels as well as EPR measurements of absorbed nitroxide probes shows that bis-BP gels are more highly crosslinked. Although we have focused on crosslinking traditional macromolecules, we expect that the methodology is suitable for stabili- zing and networking a variety of systems including emerging materials based on self-assembly and supramolecular chemistry.40,41

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