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Effect of monomer structure on radiation grafting of charge transfer complexes to synthetic and naturally occurring polymers
Radiation Physics and Chemistry 62 (2001) 89–98
Effect of monomer structure on radiation grafting of charge transfer complexes to synthetic and naturally occurring polymers Loo-Teck Nga,*, John L. Garnettb, Elvis Zilica, Duc Nguyena a
School of Science, Food and Horticulture, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia b RadTech Australia Inc., University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia
Abstract Effect of monomer structure in photografting charge transfer (CT) complexes to typical substrates like wool, cellulose and polypropylene is reported. The importance of photoinitiators in these processes is examined. Maleic anhydride (MA) with triethylene glycol divinyl ether (DVE-3) is used as reference CT complex in this work. The additional monomers studied include the esters of MA as acceptors and vinyl acetate as donor. The role of solvent in these reactions is discussed, particularly the effect of aromatics in photografting to naturally occurring trunk polymers like wool and cellulose. The effect of the double bond molar ratio of the DA components in grafting is examined. The ultraviolet (UV) conditions for gel formation during photografting, hence the importance of homopolymer yields in these processes is reported. A plausible mechanism to explain the results from this photografting work is proposed. The significance of these photografting studies in the related field of curing, especially in UV and ionising radiation systems, is discussed. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: UV; CT complexes; Grafting; Substrates; Solvent effect
1. Introduction The conventional photografting of polymers onto substrates normally involves the use of photoinitiators (PIs). These reagents besides being costly, may give low molecular weight residues, and the possible release of these residues from the finished products into the environment is of considerable concern. The advent of charge transfer (CT) complexes for both grafting and curing reactions potentially introduces a new field of ultraviolet (UV) self-sensitising processes which generate initiating species for grafting to substrates. These processes are economical to operate, rapid, pollution free and environmentally friendly. Significant work has now been performed on the mechanisms of these polymerisation processes and the results have been well *Corresponding author. Tel.: +61-2-4736-0440; fax: +61-24736-0457. E-mail address: [email protected] (L.-T. Ng).
documented (Jo. nsson et al., 1997, 1998; Ericsson et al., 2000; Garnett et al., 2000b). Advantages associated with using such a process are accentuated on this being a PI free system which results in: (i) the minimisation of oxygen inhibition during the polymerisation process, (ii) the elimination of residual fragments from the initiator left in the polymerised products, and (iii) improved economics. In recent years, the present authors have published several papers related to CT complexes in concurrent grafting and curing processes (Garnett et al., 1999a,b, 2000a,b). The essential difference between grafting and curing is the nature of bonding. In grafting, covalent carbon–carbon bonds are formed between the cured polymeric product and the substrate, therefore sites must be formed on the substrates before grafting can occur. In curing, bonding involves essentially weaker Van der Waals or London dispersion forces between the substrate and the polymeric coating. The role of CT complexes in grafting can be of significance in the following three aspects: (i) CT
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 2 5 - X
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complexes can act as additives in accelerating the grafting of monomers/oligomers particularly acrylates which are the generic type of monomers predominantly used in curing, (ii) individual components of the CT complexes, e.g. vinyl ethers are useful additives in enhancing the reactivity of monomers such as methyl methacrylate (MMA), and (iii) CT complexes can be directly grafted onto substrates. This paper focuses on the direct grafting of CT complexes to representative synthetic and naturally occurring polymers. In particular, the effect of varying the monomer components in the CT complex on the photografting yields has been examined using cellulose, wool and polypropylene (PPE) as representative substrates. The significance of this UV work in the related fields of curing and grafting initiated by ionising radiation sources like EB are also discussed.
2. Experimental 2.1. Materials Maleic anhydride (MA), the maleates and vinyl acetate (VA) were purchased from Sigma Aldrich with DVE-3 from ISP used as received. Solvents of analytical grade were used without further purification. Strips of cellulose (Whatman No 41 acid washed chromatography filter paper), PPE (isotactic film, 0.10 cm thickness) and wool with thickness of 200 mm (Belmerino quality supplied by Geelong Labs Australia) were pretreated prior to addition to reacting solution according to the method previously described (Garnett et al., 1999b). 2.2. Procedure The monomer components of the CT complex were mixed on a molar basis according to their double bond functionality unless otherwise specified in the Figures or Tables. Thus for MA:DVE-3 the mole ratio of 2 : 1 was used based on the bifunctionality of the donor. Reacting solutions (159) were prepared by dissolving monomer
components in various solvents to achieve 20%, 40%, 60% and 80% concentrations. The pretreated substrates were fully immersed in the reacting solutions and irradiated with a 90 W medium pressure mercury lamp at room temperature. The substrate samples were exposed at 30 cm from the lamp at a dose rate of 36 J h1 for the required dose as shown in the Figures and Tables, and uranyl oxalate actinometry was the method adopted for the calibration of the dose rate. At the completion of the irradiation, samples were exhaustively extracted to constant weight as previously described (Viengkhou et al., 1997).
3. Results and discussion 3.1. Effect of structure of backbone polymer on grafting The structure of the trunk polymer can significantly affect the efficiency of grafting especially with very polar materials like cellulose and wool. Cellulose is a linear polymer built by the combination of anhydroglucose repeating units as shown in Fig. 1. Cellulose is not water soluble due to the immense size of the polymeric chain involved. When it is exposed to UV light, oxidative reactions are initiated and free radicals are formed leading ultimately to grafting if monomers are present (reactions 1 and 2). hv
CeO H - CeOd þ Hd ;
ð1Þ
CeOd þ nM-CeO ðMÞn1 Md :
ð2Þ
Even though the backbone of cellulose is highly structured or bulky, in the presence of an appropriate solvent, swelling of the backbone may take place. This will allow diffusion and therefore an increase in the mobility of radicals generated in the monomer under the influence of an irradiant source, to the active sites of the substrate backbone. An enhancement in the initiation of the grafting reaction can therefore occur. Wool is essentially a protein (keratin) and is composed of eighteen amino acid residues. The bonding
Fig. 1. Cellulose structure.
L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98
between the amino residues, the cystine linkages and intramolecular hydrogen bonding are responsible for the shaping and setting characteristics of wool fibre (Fig. 2). As wool absorbs UV light effectively, degradation occurs at the disulfide bonds, where the formation of the oxidised SO3 and the reduced SH species from cystine are generated. The photochemical cleavage of cystine crosslinks near the fibre surface results in soluble proteins being released from the irradiated fibres and, at the same time increases the number of bonding sites in the wool structure where grafting of monomers may occur. Solvents that possess the ability to swell the truck polymer will therefore assist in the diffusion and penetration of monomers to these active sites. Due to the polar nature of wool, it is expected that polar monomers would graft easily onto this substrate. In contrast to cellulose and wool, PPE being a hydrocarbon, is relatively non polar thus polar solvents suitable for wetting and swelling the two naturally
91
occurring trunk polymers are not necessarily applicable to PPE. This situation is saved by the presence of many monomers like styrene which can swell PPE and thus enable polar solvents to penetrate the structure. 3.2. Solvent effects in grafting CT complexes Previous reports (Garnett et al., 1999a, 2000a) have demonstrated the importance of solvents in grafting reactions involving conventional monomers initiated by UV and ionising radiation. Preliminary studies of analogous effects in CT processes have indicated that solvents are important in such DA reactions (Garnett et al., 1999c). As expected, in the latter reactions, solvents which wet and swell the substrates can influence the grafting reactivity. In addition, in CT work, solvents can interact with the donor or acceptor to yield intermediates possessing unique charge transfer properties.
Fig. 2. Wool structure.
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Fig. 3. Photografting of MA/DVE-3 to cellulose without PI in various solvents. 25 J at 36 J h1. Temperature 201C; double bond molar ratio MA/DVE-3 (1.0/2.0). (+), DCM; ( ), CCl4; (m), ethyl acetate; (E), acetonitrile; (’), THF; (K), acetone; for comparison of grafting without solvent 585% at 34 J dose; grafting solution 15 g.
Fig. 4. Photografting of MA/DVE-3 to cellulose with PI in various solvents. PI, Irgacure 1800 (0.05% w/w). Temperature 201C; dose 7 J; other conditions as in Fig. 1; (+), DCM; ( ), CCl4; (m), EtAc; (E), ACN; (’), THF; (K), acetone; for comparison of grafting without solvent 34% at 4 J (Irgacure 1800, 0.1% w/w); grafting solution 15 g.
More detailed studies of solvent effects in CT work are the data reported in Fig. 3 for the photografting of MA/DVE-3 to cellulose. In the absence of PI, grafting can actually be enhanced at certain concentrations in particular solvents, dichloromethane (DCM) being very effective in this respect at all monomer concentrations studied. With the inclusion of PI (Fig. 4) grafting yields are strongly enhanced when compared with the corresponding data in Fig. 3 as demonstrated not only by the numerical grafting values but also by the much lower UV doses (7 versus 25 J at constant solution weight of 15 g) required to achieve these data. The result illustrates the effect of free radical formation from the PI in yielding more sites in the
substrates where grafting may occur (reactions 3 and 4). hv
PI - Pd þ Id ; hv
Pd þ CeOH - CeOd þ PH:
ð3Þ ð4Þ
This conclusion is consistent with the dichloromethane results in Fig. 3 where additional radicals can be formed by reaction 5. hv
CH2 CI2 - CH2 CId þ CId :
ð5Þ
A plausible mechanism whereby the CT complex DA (represented by DVE-3 and MA respectively) is grafted to substrate SH is shown in
L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98
reactions 6–8. hv
D þ A-½D A - ½D A * -Rd ; hv
SH - Sd þ Hd ;
ð6Þ ð7Þ
Sd þ Rd -Graft:
ð8Þ d
A process for the formation of R has been discussed (Decker et al., 1997). A path for radical generation from DA is shown in Scheme 1 where photopolymerisation of MA:DVE-3 leads to the formation of an exciplex containing the cyclobutane ring which decomposes to the 1,4-biradical species which can be active in the grafting process. An additional advantage of the solvents used in Figs. 3 and 4 is that most of them are extremely polar, thus dissolving the homopolymer as it is formed and hence facilitating removal of grafted substrate at the conclusion of the reaction. Precipitation of homopoly-
93
mer in these photografting reactions can also, in certain solvents, lead to turbidity in the supernatant grafting solution thus restricting transmission of UV through the supernatant and terminating the polymerisation process. A further interesting feature of the data in Figs. 3 and 4 is that grafting is generally optimised at the highest monomer concentrations studied. This result is consistent with the fact that this is the concentration where the solvent has least dilution effect on DA formation in the series investigated. As a consequence the frequency of D and A interacting to give DA is increased at the highest monomer concentrations compared to the low 20% v/v solutions. When cellulose is replaced by wool as substrate, grafting yields of MA:DVE-3 with wool are marginally lower under the experimental conditions in Table 1. This difference may be attributed to the presence in the cellulose structure of labile acetal hydrogens which are easily abstractable, therefore leading to an increase in grafting sites. Another factor for lower grafting
Scheme 1. Formation of 1,4-biradical species.
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Table 1 Photografting of MA/DVE-3 onto woola Solvents
THF Methanol Toluene Ethyl acetate CCl4 CH2Cl2 CHCl3 Acetonitrile Acetone a
Graft (%) at monomer concentration (%w/w) 80%
Dose (J)
60%
Dose (J)
40%
Dose (J)
20%
Dose (J)
63 37 53 75 67 34 59 29 35
35 162 84 76 37 162 162 162 48
32 24 29 78 18 12 15 31 23
162 162 162 162 162 162 162 162 162
15 32 921 23 10 16 7 19 19
162 162 162 162 162 162 162 162 162
8 39 6 13 8 10 7 9 14
162 162 162 162 162 162 162 162 162
Temperature 201C; MA:DVE-3 (equimolar); dose rate 36 J h1; blanks without UV yielded no graft.
Fig. 5. Effect of maleate esters as acceptors with donor DVE-3 in photografting to cellulose in chloroform. Without PI (- - -) dose 73 J, with PI (F) dose 4 J. (K), mono-2-(methacryloxy) ethyl maleate; (m), mono-2-ethyl hexyl maleate; (E), bis (2-ethyl hexyl) maleate; ( ), mono-butyl maleate; other conditions as in Figs. 3 and 4.
results with wool may be the fact that wool has extensive interlinkages across the structure (Fig. 2) involving functional groups tightly bonded and packed into a rigid structure by the carbon nitrogen and intermolecular hydrogen bonding. The radicals formed in the course of CT complex formation may then take part in extensive homopolymerisation instead thus limiting the grafting yield with this substrate. The other interesting point concerning this wool data is the result when toluene is used as solvent. Although aromatic in nature, toluene still results in a reasonable grafting yield especially at the highest monomer concentrations studied, thus suggesting that DVE-3 is the swelling agent in this instance. 3.3. Grafting with maleate esters as acceptors The use of MA as a component in the DA charge transfer complex in these studies possesses a number of disadvantages especially when used without solvent.
Thus there can be solubility problems and also difficulty with the speed of the reaction. In some experiments, e.g. neat monomers in bulk, reaction speed can be extremely fast whereas under other conditions such as coating applications the reverse may be observed. In addition grafted MA may hydrolyse in the copolymer to alter the properties of the finished product. The use of maleate esters (Figs. 5 and 6) overcomes some of these difficulties. Consistent with the MA/DVE3 results the data in Fig. 5 for the corresponding ester/ DVE-3 systems for grafting to cellulose in chloroform show a maximum in graft at the highest monomer concentrations studied, namely 80% v/v. Inclusion of PI in these ester systems again enhances graft considerably as expected. When compared with the analogous MA/ DVE-3 chloroform data previously reported, the reactivity of the ester systems is lower than that of the corresponding MA complex, suggesting that both steric and electronic effects of the maleates influence subsequent reactivity of these compounds.
L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98
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Fig. 6. Effect of maleate esters as acceptors with donor DVE-3 in photografting to cellulose in aromatic solvent benzene. Without PI (- - -) dose 53 J, with PI (F) dose 7 J. (K), mono-2-(methacryloxy) ethyl maleate; (m), mono-2-ethylhexyl maleate; (E), bis (2-ethyl hexyl) maleate; ( ), mono-butyl maleate; other conditions as in Figs. 3 and 4.
Table 2 Photografting of MA/DVE-3 to cellulose with benzene as solventa Monomer
80 60 40 20 a
No PI
PI
Dose (J)
Graft (%)
Dose (J)
Graft (%)
37 37 37 37
2.9 2.3 4 5.3
1.2 1.2 3.6 3.6
400 157 52 21
Irgacure 1800 (0.05% w/w); other conditions as in Table 1.
The results of the grafting reactions to cellulose when benzene is used as solvent in Fig. 6 are interesting since this solvent does not readily wet and swell the trunk polymer. This has been the reason for the poor reactivity of monomers in benzene with this backbone polymer in previous radiation grafting work. The present results with the esters and cellulose, especially with PI present, show clearly that at relatively low UV doses, very efficient grafting of the DA complex to cellulose occurs in benzene. This result suggests that DVE-3 is the active component which is swelling the cellulose and leading to grafting of the CT complex. In addition, the esters themselves may be contributing to this reaction. Once the cellulose has swollen, radicals from the PI can then accentuate the grafting process. When the reactivity of the ester complexes is compared with the corresponding data for the grafting of MA/DVE-3 in benzene to cellulose (Table 2) the MA:DVE-3 complex is significantly more reactive as evidenced by the UV doses required to achieve comparable grafting (1.2 J versus 7.0 J for the esters at constant solution weight of 15 g). These results again suggest that electronic and steric effects of the substituents influence the polymerisation patterns of the esters.
3.4. Effect of double bond molar ratio (DBMR) of DA monomers on grafting In conventional radiation curing, the DBMR of the donor and acceptor in the CT complex would be expected to be close to 1.0/1.0 for efficient reactivity. In grafting work however, there may be a significant deviation from this statistical ratio since especially in polar backbone polymers like cellulose, preferential absorption of polar DVE-3 into the substrate could be expected thus significantly altering the above ratio. In the relevant DBMR results for the MA/DVE-3 system (Table 3), for the neat monomers in the absence of PI, significantly higher doses of UV are needed for the 1.0/ 1.5 (MA/DVE-3) system compared with the statistical ratio to achieve significant grafting. When additives are included in the system, either PI or solvents, the situation becomes more complicated since solvents may themselves complex with the donor or acceptor and the PI itself contributes additional radicals to the process. In the presence of these additives the DBMR is not as critical as with the neat monomers. When dimethyl maleate (DMMA) replaces MA in the DVE-3 complex, even with the neat monomers, the DBMR again does not appear to be as important. This result reflects the fact that the DMMA system is much slower to react than the MA complex as evidenced by the higher UV doses needed when compared with MA/ DVE-3 to achieve reaction especially in the neat 1.0/1.0 complexes. These conclusions are consistent for both PPE and cellulose.
3.5. Grafting of VA as donor with MA DVE-3 is a relatively expensive monomer and, if CT work is to be commercialised, lower cost donors would be an incentive. In this respect VA has potential
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Table 3 Significance of double bond molar ratio (DBMR) of DA components on photografting CT complexes in THF solventa CT complex
DBMR
MA/DVE-3
1.0/1.0
DMMA/DVE-3
1.0/1.0
MA/DVE-3
1.0/1.5
DMMA/DVE-3
1.0/1.5
a
Substrate
Solvent
No PI
PI
Dose (J)
Graft (%)
Dose (J)
Graft (%)
PPE Cellulose PPE Cellulose
0 0 THF THF
3 13 150 180
60 200 6 230
2 2 7 7
70 380 2 160
PPE Cellulose PPE Cellulose
0 0 THF THF
123 160 150 42
19 230 13 142
7 7 9 13
14 480 6 220
PPE Cellulose PPE Cellulose
0 0 THF THF
113 113 180 180
39 230 49 320
4 4 4 7
7 830 12 430
PPE Cellulose PPE Cellulose
0 0 THF THF
114 128 184 184
14 240 5 170
10 10 4 4
69 480 61 300
Monomer concentration (90% v/v); PI (if present), Irgacure 1800 (0.05% w/w); other conditions as in Table 1.
Table 4 Vinyl acetate as donor with acceptor maleic anhydride in photografting to PPE and cellulose in acetone solvent Substrate
PIa (%)
Monomer conc (%v/v) 80
PPE PPE Cellulose Cellulose a
1 0 1 0
60
40
20
Dose (J)
Graft (%)
Dose (J)
Graft (%)
Dose (J)
Graft (%)
Dose (J)
Graft (%)
37 220 37 220
8 0 26 6
110 220 110 220
14 0.1 5 6
220 220 110 220
5 1 1 8
220 220 110 220
9 0.4 3 6
Irgacure 1800 (1.0% w/w); DBMR, MA/VA (1.0/1.0); other conditions as in Table 1.
although the data in Table 4 for the photografting of MA/VA to both PPE and cellulose show that VA is much less reactive than DVE-3 in this work even with the incorporation of PI in the system. This situation is improved when DVE-3 replaces half the VA on a DBMR basis (Table 5), benzene being a particularly useful solvent in this respect with cellulose as substrate. The interpretation of these data is complicated by the fact that there are two donors with the one acceptor in the system, however the results show that VA has potential in this field.
Table 5 Vinyl acetate and DVE-3 as comonomer donors with acceptor maleic anhydride in photografting to cellulosea Solvent
Chloroform Toluene Benzene DMF THF
No PI
PI
Dose (J)
Graft (%)
Dose (J)
Graft (%)
24 24 24 24 24
4 0 266 5 0
3.7 3.7 3.7 3.7 3.7
0 13 11 43 0
a Monomer concentration (80% v/v); temperature 241C; DBMR, MA/VA/DVE-3 (1.0/0.5/0.5); other conditions as in
3.6. Role of gel pointFnature of copolymer As with any radiation grafting system, the role of homopolymer is important. In this respect the grafting of CT complexes is no exception. The CT system is
however more complicated than most conventional monomer radiation processes. In the former case, grafting in the presence of solvent is fastest at the highest monomer concentrations whereas with conven-
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L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98 Table 6 UV conditions for photografting of MA/DVE-3 to cellulose without PI at gel point Monomer conc (%v/v) Solvent
Acetone CHCl3 CH2Cl2 THF
80
60 a
40 a
20 a
Dose (J)
Graft (%)
P
Dose (J)
Graft (%)
P
Dose (J)
Graft (%)
P
Dose (J)
Graft (%)
Pa
494 358 358 428
44 83 113 119
Ge Ge Ge Ge
494 428 358 494
19 68 51 53
N.Ge Ge Ge Ge
494 494 494 494
11 62 46 38
N.Ge Ge N.Ge N.Ge
494 494 494 494
8 66 17 8
N.Ge N.Ge N.Ge N.Ge
a P=polymer supernatant gelled (Ge); N.Ge=no gel; DBMR, MA/DVE-3 (1.0/1.0); temperature 231C; other conditions as in Table 1.
Table 7 UV conditions for photografting of MA/DVE-3 to cellulose with PI at gel point Monomer conc (%v/v) Solvent
Acetone CHCl3 CH2Cl2 THF a
80
60
40
20
Dose (J)
Graft (%)
Pa
Dose (J)
Graft (%)
Pa
Dose (J)
Graft (%)
Pa
Dose (J)
Graft (%)
Pa
5 5 5 5
42 43 52 396
Ge Ge Ge Ge
6 5 5 5
24 65 89 82
Ge Ge Ge Ge
6 5 5 6
51 27 75 31
Ge Ge Ge Ge
6 6 5 5
20 24 13 23
Ge Ge Ge N.Ge
Irgacure 1800 (0.05% w/w); other conditions as in Table 6.
tional monomer systems grafting tends to reach a peak at much lower monomer concentrations thus facilitating handling of the experimental system because of lower viscosity. More importantly, as the supernatant liquid in the CT system increases in viscosity due to homopolymer formation, the homopolymer formed tends to crosslink especially in the presence of bifunctional monomers like DVE-3 yielding a gel from which it can be difficult to extract the graft copolymer. For this reason in much of the current work the systems are taken almost to gel point, then the grafted substrate is removed. The data in Tables 6 and 7 illustrate this point. In Table 6 where MA/DVE-3 is photografted to cellulose without PI, the UV dose required to almost attain gel point in the supernatant varies considerably with the structure of solvent especially at the highest monomer concentrations studied. Consistent with earlier solvent experiments discussed in this grafting work, halogenated solvents are the most reactive and require the lowest UV doses to gel, this observation reflecting the ease with which radicals can be formed in these solvents upon exposure to UV. Inclusion of PI in these systems has a dramatic effect on the dose to gel (Table 7). Speed of polymerisation is now so fast, due to the additional radicals from the PI, that there is little difference in dose to gel amongst the various solvents, the dose being so low (5 J). Under the experimental conditions in Table 7, at the highest
monomer concentration studied in THF, the grafting yields are extremely high, a result which should be of value in preparative work. A further important aspect of the present CT photografting work is the nature of the graft copolymer formed. At high monomer concentrations, in appropriate CT systems, the grafting yield increases strongly as the polymerisation in the supernatant approaches gel point. Under these conditions, the polymer system tends to be severely crosslinked especially if multifunctional monomers are involved as one of the DA components. The difficulty of such a situation is that as the reaction reaches the gel point it can be difficult to remove graft copolymer. Care thus needs to be exercised in removing the grafted substrate prior to reaching gel. A further significant feature of the current work is that in addition to graft copolymers, the present system is of value in the synthesis of related IPNS and semi IPNS. The present authors are synthesising these materials from CT complexes. At this time one of the important problems in this grafting field is to characterise in detail the structures of the CT graft copolymers. Work on this problem is currently in progress, especially designed to overcome (i) crosslinking due to the presence of at least one binary functional component in the DA complex and (ii) the use of certain solvents which produce high radical yields that can complicate interpretation of the grafting process.
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3.7. Significance in curing The present data are relevant to analogous radiation curing processes. The type of monomers used in this CT monomer work are the same, or similar to, those used in UV curing. The present results illustrate the type of UV conditions where grafting can be expected to occur during the UV curing process. This observation is important in curing since the onset of concurrent grafting enhances the adhesion between the cured film and substrate and thus restricts the possibility of delamination occurring in the final product. This property is particularly relevant to naturally occurring macromolecules like cellulose and wool since, with these substrates in some applications, the presence of grafting during cure can render difficult the ultimate recycling of the finished product. In contrast this recycling problem with coated plastics like the polyolefins is not such a problem since these finished grafted products as waste materials can be relatively easily utilised elsewhere. Thus for some applications the presence of concurrent grafting in the final product is an advantage whereas in others it can be an impediment. The type of experimental UV conditions where such grafting may be observed are thus useful and are demonstrated in this paper. The present UV grafting work is also a guide to curing systems using ionising radiation like EB processes since previous work (Garnett et al., 1999b) has shown that UV and ionising radiation grafting systems are closely related. The radiation conditions required to achieve grafting established through UV can now be readily extrapolated to ionising radiation systems with the reverse process also possible.
4. Conclusions Polar solvents, especially the halogenated derivatives like DCM are shown to be effective in the photografting of a typical CT complex, MA/DVE-3, to representative trunk polymers like cellulose, wool and PPE. Aromatic solvents are also suitable for these processes even with polar substrates like cellulose and wool. Inclusion of appropriate PIs in the monomer solution enhances the grafting yield considerably, leading to lower UV doses to achieve a particular percentage graft. Photografting yields are highest at the highest monomer concentrations studied. Esterification of MA yields compounds which readily graft to cellulose as CT complexes but with lower efficiency than MA. The DBMR ratio of 1.0 : 1.0 is shown not to be critical for the photografting of complexes analogous to MA:DVE-3. VA is also a suitable donor in these processes with MA, although less reactive than DVE-3. Gel formation readily occurs with the more reactive CT complexes at relatively low UV doses. Mechanistically, the observed data are consistent
with the participation of a CT complex formed between the grafting monomers and active sites in the trunk polymer. The current photografting work is shown to be of value in the related fields of UV and EB curing.
Acknowledgements The authors thank AINSE and Ballina Pty Ltd for support.
References Decker, C., Morel, F., Jo. nsson, S., Clark, S., Hoyle, C., 1997. UV radiation curing of photoinitiator free formulations. Proceedings of the Rad Tech. Europe ’97, Academic Days, Lyon, p. 169. Ericsson, J., Nilsson, M., Lundmark, S., Svensson, L., Jo. nsson, S., Lindgren, K., 2000. Synthesis and photoinduced copolymerisation of novel maleimides, combined with vinyland/or styrene ethers. Proceedings of the Rad Tech. North America 2000, Baltimore, p. 173. Garnett, J.L., Ng, L.-T., Viengkhou, V., 1999a. Grafting of methyl methacrylate to cellulose and polypropylene with UV and ionising radiation in the presence of additives including CT complexes. Radiat. Phys. Chem. 56, 387–403. Garnett, J.L., Ng, L.-T., Viengkhou, V., Zilic, E., 1999b. Novel additives including charge-transfer complexes in grafting of methyl methacrylate to polypropylene, cellulose and wool, initiated by ionising radiation and UV: significance of these studies in analogous curing processes. Polym. Int. 48, 1016–1026. Garnett, J.L., Ng, L.-T., Viengkhou, V., Hennessy, I., Shah, N.H., Zilic, E.F., 1999c. Solvent effects in photografting MMA and CT complexes to cellulose, wool and polypropylene. Significance of studies in radiation curing. Proceedings of the Rad Tech. Europe 1999, Berlin, p. 677. Garnett, J.L., Ng, L.-T., Viengkhou, V., Hennessy, I.V., Zilic, E.F., 2000a. Significance of grafting in curing processes initiated by UV, excimer and ionising radiation sources. Radiat. Phys. Chem. 57, 355–359. Garnett, J.L., Ng, L.-T., Viengkhou, V., Hennessy, I.V., Zilic, E.F., 2000b. Photoinitator free UV grafting and curing using CT complexes on polypropylene: comparison with cellulose and wool as substrates. Proceedings of the Rad Tech. North America 2000, Baltimore, USA, p. 804. Jo. nsson, S., Sundell, P-E., Shimose, M., Clark, S., Miller, C., Morel, F., Decker, C., Hoyle, C., 1997. Photo-induced alternating copolymerisation of N-substituted maleimides and electron donor olefins. Nucl. Instrum. Methods B 131, 276. Jo. nsson, S., Hasselgren, C., Ericsson, J.S., Johanson, M., Clark, S., Miller, C., Hoyle, C., 1998. Oxygen accelerator effects in copolymerisation of donor-acceptor pairs. Proceedings of the Rad Tech. North America ‘98, Chicago, p. 189. Viengkhou, V., Ng, L.-T., Garnett, J.L., 1997. The effect of additives on the enhancement of methyl methacrylate grafting to cellulose in the presence of UV and ionising radiation. Radiat. Phys. Chem. 49, 595–602.
Effect of monomer structure on radiation grafting of charge transfer complexes to synthetic and naturally occurring polymers Loo-Teck Nga,*, John L. Garnettb, Elvis Zilica, Duc Nguyena a
School of Science, Food and Horticulture, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia b RadTech Australia Inc., University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia
Abstract Effect of monomer structure in photografting charge transfer (CT) complexes to typical substrates like wool, cellulose and polypropylene is reported. The importance of photoinitiators in these processes is examined. Maleic anhydride (MA) with triethylene glycol divinyl ether (DVE-3) is used as reference CT complex in this work. The additional monomers studied include the esters of MA as acceptors and vinyl acetate as donor. The role of solvent in these reactions is discussed, particularly the effect of aromatics in photografting to naturally occurring trunk polymers like wool and cellulose. The effect of the double bond molar ratio of the DA components in grafting is examined. The ultraviolet (UV) conditions for gel formation during photografting, hence the importance of homopolymer yields in these processes is reported. A plausible mechanism to explain the results from this photografting work is proposed. The significance of these photografting studies in the related field of curing, especially in UV and ionising radiation systems, is discussed. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: UV; CT complexes; Grafting; Substrates; Solvent effect
1. Introduction The conventional photografting of polymers onto substrates normally involves the use of photoinitiators (PIs). These reagents besides being costly, may give low molecular weight residues, and the possible release of these residues from the finished products into the environment is of considerable concern. The advent of charge transfer (CT) complexes for both grafting and curing reactions potentially introduces a new field of ultraviolet (UV) self-sensitising processes which generate initiating species for grafting to substrates. These processes are economical to operate, rapid, pollution free and environmentally friendly. Significant work has now been performed on the mechanisms of these polymerisation processes and the results have been well *Corresponding author. Tel.: +61-2-4736-0440; fax: +61-24736-0457. E-mail address: [email protected] (L.-T. Ng).
documented (Jo. nsson et al., 1997, 1998; Ericsson et al., 2000; Garnett et al., 2000b). Advantages associated with using such a process are accentuated on this being a PI free system which results in: (i) the minimisation of oxygen inhibition during the polymerisation process, (ii) the elimination of residual fragments from the initiator left in the polymerised products, and (iii) improved economics. In recent years, the present authors have published several papers related to CT complexes in concurrent grafting and curing processes (Garnett et al., 1999a,b, 2000a,b). The essential difference between grafting and curing is the nature of bonding. In grafting, covalent carbon–carbon bonds are formed between the cured polymeric product and the substrate, therefore sites must be formed on the substrates before grafting can occur. In curing, bonding involves essentially weaker Van der Waals or London dispersion forces between the substrate and the polymeric coating. The role of CT complexes in grafting can be of significance in the following three aspects: (i) CT
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 2 5 - X
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complexes can act as additives in accelerating the grafting of monomers/oligomers particularly acrylates which are the generic type of monomers predominantly used in curing, (ii) individual components of the CT complexes, e.g. vinyl ethers are useful additives in enhancing the reactivity of monomers such as methyl methacrylate (MMA), and (iii) CT complexes can be directly grafted onto substrates. This paper focuses on the direct grafting of CT complexes to representative synthetic and naturally occurring polymers. In particular, the effect of varying the monomer components in the CT complex on the photografting yields has been examined using cellulose, wool and polypropylene (PPE) as representative substrates. The significance of this UV work in the related fields of curing and grafting initiated by ionising radiation sources like EB are also discussed.
2. Experimental 2.1. Materials Maleic anhydride (MA), the maleates and vinyl acetate (VA) were purchased from Sigma Aldrich with DVE-3 from ISP used as received. Solvents of analytical grade were used without further purification. Strips of cellulose (Whatman No 41 acid washed chromatography filter paper), PPE (isotactic film, 0.10 cm thickness) and wool with thickness of 200 mm (Belmerino quality supplied by Geelong Labs Australia) were pretreated prior to addition to reacting solution according to the method previously described (Garnett et al., 1999b). 2.2. Procedure The monomer components of the CT complex were mixed on a molar basis according to their double bond functionality unless otherwise specified in the Figures or Tables. Thus for MA:DVE-3 the mole ratio of 2 : 1 was used based on the bifunctionality of the donor. Reacting solutions (159) were prepared by dissolving monomer
components in various solvents to achieve 20%, 40%, 60% and 80% concentrations. The pretreated substrates were fully immersed in the reacting solutions and irradiated with a 90 W medium pressure mercury lamp at room temperature. The substrate samples were exposed at 30 cm from the lamp at a dose rate of 36 J h1 for the required dose as shown in the Figures and Tables, and uranyl oxalate actinometry was the method adopted for the calibration of the dose rate. At the completion of the irradiation, samples were exhaustively extracted to constant weight as previously described (Viengkhou et al., 1997).
3. Results and discussion 3.1. Effect of structure of backbone polymer on grafting The structure of the trunk polymer can significantly affect the efficiency of grafting especially with very polar materials like cellulose and wool. Cellulose is a linear polymer built by the combination of anhydroglucose repeating units as shown in Fig. 1. Cellulose is not water soluble due to the immense size of the polymeric chain involved. When it is exposed to UV light, oxidative reactions are initiated and free radicals are formed leading ultimately to grafting if monomers are present (reactions 1 and 2). hv
CeO H - CeOd þ Hd ;
ð1Þ
CeOd þ nM-CeO ðMÞn1 Md :
ð2Þ
Even though the backbone of cellulose is highly structured or bulky, in the presence of an appropriate solvent, swelling of the backbone may take place. This will allow diffusion and therefore an increase in the mobility of radicals generated in the monomer under the influence of an irradiant source, to the active sites of the substrate backbone. An enhancement in the initiation of the grafting reaction can therefore occur. Wool is essentially a protein (keratin) and is composed of eighteen amino acid residues. The bonding
Fig. 1. Cellulose structure.
L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98
between the amino residues, the cystine linkages and intramolecular hydrogen bonding are responsible for the shaping and setting characteristics of wool fibre (Fig. 2). As wool absorbs UV light effectively, degradation occurs at the disulfide bonds, where the formation of the oxidised SO3 and the reduced SH species from cystine are generated. The photochemical cleavage of cystine crosslinks near the fibre surface results in soluble proteins being released from the irradiated fibres and, at the same time increases the number of bonding sites in the wool structure where grafting of monomers may occur. Solvents that possess the ability to swell the truck polymer will therefore assist in the diffusion and penetration of monomers to these active sites. Due to the polar nature of wool, it is expected that polar monomers would graft easily onto this substrate. In contrast to cellulose and wool, PPE being a hydrocarbon, is relatively non polar thus polar solvents suitable for wetting and swelling the two naturally
91
occurring trunk polymers are not necessarily applicable to PPE. This situation is saved by the presence of many monomers like styrene which can swell PPE and thus enable polar solvents to penetrate the structure. 3.2. Solvent effects in grafting CT complexes Previous reports (Garnett et al., 1999a, 2000a) have demonstrated the importance of solvents in grafting reactions involving conventional monomers initiated by UV and ionising radiation. Preliminary studies of analogous effects in CT processes have indicated that solvents are important in such DA reactions (Garnett et al., 1999c). As expected, in the latter reactions, solvents which wet and swell the substrates can influence the grafting reactivity. In addition, in CT work, solvents can interact with the donor or acceptor to yield intermediates possessing unique charge transfer properties.
Fig. 2. Wool structure.
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Fig. 3. Photografting of MA/DVE-3 to cellulose without PI in various solvents. 25 J at 36 J h1. Temperature 201C; double bond molar ratio MA/DVE-3 (1.0/2.0). (+), DCM; ( ), CCl4; (m), ethyl acetate; (E), acetonitrile; (’), THF; (K), acetone; for comparison of grafting without solvent 585% at 34 J dose; grafting solution 15 g.
Fig. 4. Photografting of MA/DVE-3 to cellulose with PI in various solvents. PI, Irgacure 1800 (0.05% w/w). Temperature 201C; dose 7 J; other conditions as in Fig. 1; (+), DCM; ( ), CCl4; (m), EtAc; (E), ACN; (’), THF; (K), acetone; for comparison of grafting without solvent 34% at 4 J (Irgacure 1800, 0.1% w/w); grafting solution 15 g.
More detailed studies of solvent effects in CT work are the data reported in Fig. 3 for the photografting of MA/DVE-3 to cellulose. In the absence of PI, grafting can actually be enhanced at certain concentrations in particular solvents, dichloromethane (DCM) being very effective in this respect at all monomer concentrations studied. With the inclusion of PI (Fig. 4) grafting yields are strongly enhanced when compared with the corresponding data in Fig. 3 as demonstrated not only by the numerical grafting values but also by the much lower UV doses (7 versus 25 J at constant solution weight of 15 g) required to achieve these data. The result illustrates the effect of free radical formation from the PI in yielding more sites in the
substrates where grafting may occur (reactions 3 and 4). hv
PI - Pd þ Id ; hv
Pd þ CeOH - CeOd þ PH:
ð3Þ ð4Þ
This conclusion is consistent with the dichloromethane results in Fig. 3 where additional radicals can be formed by reaction 5. hv
CH2 CI2 - CH2 CId þ CId :
ð5Þ
A plausible mechanism whereby the CT complex DA (represented by DVE-3 and MA respectively) is grafted to substrate SH is shown in
L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98
reactions 6–8. hv
D þ A-½D A - ½D A * -Rd ; hv
SH - Sd þ Hd ;
ð6Þ ð7Þ
Sd þ Rd -Graft:
ð8Þ d
A process for the formation of R has been discussed (Decker et al., 1997). A path for radical generation from DA is shown in Scheme 1 where photopolymerisation of MA:DVE-3 leads to the formation of an exciplex containing the cyclobutane ring which decomposes to the 1,4-biradical species which can be active in the grafting process. An additional advantage of the solvents used in Figs. 3 and 4 is that most of them are extremely polar, thus dissolving the homopolymer as it is formed and hence facilitating removal of grafted substrate at the conclusion of the reaction. Precipitation of homopoly-
93
mer in these photografting reactions can also, in certain solvents, lead to turbidity in the supernatant grafting solution thus restricting transmission of UV through the supernatant and terminating the polymerisation process. A further interesting feature of the data in Figs. 3 and 4 is that grafting is generally optimised at the highest monomer concentrations studied. This result is consistent with the fact that this is the concentration where the solvent has least dilution effect on DA formation in the series investigated. As a consequence the frequency of D and A interacting to give DA is increased at the highest monomer concentrations compared to the low 20% v/v solutions. When cellulose is replaced by wool as substrate, grafting yields of MA:DVE-3 with wool are marginally lower under the experimental conditions in Table 1. This difference may be attributed to the presence in the cellulose structure of labile acetal hydrogens which are easily abstractable, therefore leading to an increase in grafting sites. Another factor for lower grafting
Scheme 1. Formation of 1,4-biradical species.
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Table 1 Photografting of MA/DVE-3 onto woola Solvents
THF Methanol Toluene Ethyl acetate CCl4 CH2Cl2 CHCl3 Acetonitrile Acetone a
Graft (%) at monomer concentration (%w/w) 80%
Dose (J)
60%
Dose (J)
40%
Dose (J)
20%
Dose (J)
63 37 53 75 67 34 59 29 35
35 162 84 76 37 162 162 162 48
32 24 29 78 18 12 15 31 23
162 162 162 162 162 162 162 162 162
15 32 921 23 10 16 7 19 19
162 162 162 162 162 162 162 162 162
8 39 6 13 8 10 7 9 14
162 162 162 162 162 162 162 162 162
Temperature 201C; MA:DVE-3 (equimolar); dose rate 36 J h1; blanks without UV yielded no graft.
Fig. 5. Effect of maleate esters as acceptors with donor DVE-3 in photografting to cellulose in chloroform. Without PI (- - -) dose 73 J, with PI (F) dose 4 J. (K), mono-2-(methacryloxy) ethyl maleate; (m), mono-2-ethyl hexyl maleate; (E), bis (2-ethyl hexyl) maleate; ( ), mono-butyl maleate; other conditions as in Figs. 3 and 4.
results with wool may be the fact that wool has extensive interlinkages across the structure (Fig. 2) involving functional groups tightly bonded and packed into a rigid structure by the carbon nitrogen and intermolecular hydrogen bonding. The radicals formed in the course of CT complex formation may then take part in extensive homopolymerisation instead thus limiting the grafting yield with this substrate. The other interesting point concerning this wool data is the result when toluene is used as solvent. Although aromatic in nature, toluene still results in a reasonable grafting yield especially at the highest monomer concentrations studied, thus suggesting that DVE-3 is the swelling agent in this instance. 3.3. Grafting with maleate esters as acceptors The use of MA as a component in the DA charge transfer complex in these studies possesses a number of disadvantages especially when used without solvent.
Thus there can be solubility problems and also difficulty with the speed of the reaction. In some experiments, e.g. neat monomers in bulk, reaction speed can be extremely fast whereas under other conditions such as coating applications the reverse may be observed. In addition grafted MA may hydrolyse in the copolymer to alter the properties of the finished product. The use of maleate esters (Figs. 5 and 6) overcomes some of these difficulties. Consistent with the MA/DVE3 results the data in Fig. 5 for the corresponding ester/ DVE-3 systems for grafting to cellulose in chloroform show a maximum in graft at the highest monomer concentrations studied, namely 80% v/v. Inclusion of PI in these ester systems again enhances graft considerably as expected. When compared with the analogous MA/ DVE-3 chloroform data previously reported, the reactivity of the ester systems is lower than that of the corresponding MA complex, suggesting that both steric and electronic effects of the maleates influence subsequent reactivity of these compounds.
L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98
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Fig. 6. Effect of maleate esters as acceptors with donor DVE-3 in photografting to cellulose in aromatic solvent benzene. Without PI (- - -) dose 53 J, with PI (F) dose 7 J. (K), mono-2-(methacryloxy) ethyl maleate; (m), mono-2-ethylhexyl maleate; (E), bis (2-ethyl hexyl) maleate; ( ), mono-butyl maleate; other conditions as in Figs. 3 and 4.
Table 2 Photografting of MA/DVE-3 to cellulose with benzene as solventa Monomer
80 60 40 20 a
No PI
PI
Dose (J)
Graft (%)
Dose (J)
Graft (%)
37 37 37 37
2.9 2.3 4 5.3
1.2 1.2 3.6 3.6
400 157 52 21
Irgacure 1800 (0.05% w/w); other conditions as in Table 1.
The results of the grafting reactions to cellulose when benzene is used as solvent in Fig. 6 are interesting since this solvent does not readily wet and swell the trunk polymer. This has been the reason for the poor reactivity of monomers in benzene with this backbone polymer in previous radiation grafting work. The present results with the esters and cellulose, especially with PI present, show clearly that at relatively low UV doses, very efficient grafting of the DA complex to cellulose occurs in benzene. This result suggests that DVE-3 is the active component which is swelling the cellulose and leading to grafting of the CT complex. In addition, the esters themselves may be contributing to this reaction. Once the cellulose has swollen, radicals from the PI can then accentuate the grafting process. When the reactivity of the ester complexes is compared with the corresponding data for the grafting of MA/DVE-3 in benzene to cellulose (Table 2) the MA:DVE-3 complex is significantly more reactive as evidenced by the UV doses required to achieve comparable grafting (1.2 J versus 7.0 J for the esters at constant solution weight of 15 g). These results again suggest that electronic and steric effects of the substituents influence the polymerisation patterns of the esters.
3.4. Effect of double bond molar ratio (DBMR) of DA monomers on grafting In conventional radiation curing, the DBMR of the donor and acceptor in the CT complex would be expected to be close to 1.0/1.0 for efficient reactivity. In grafting work however, there may be a significant deviation from this statistical ratio since especially in polar backbone polymers like cellulose, preferential absorption of polar DVE-3 into the substrate could be expected thus significantly altering the above ratio. In the relevant DBMR results for the MA/DVE-3 system (Table 3), for the neat monomers in the absence of PI, significantly higher doses of UV are needed for the 1.0/ 1.5 (MA/DVE-3) system compared with the statistical ratio to achieve significant grafting. When additives are included in the system, either PI or solvents, the situation becomes more complicated since solvents may themselves complex with the donor or acceptor and the PI itself contributes additional radicals to the process. In the presence of these additives the DBMR is not as critical as with the neat monomers. When dimethyl maleate (DMMA) replaces MA in the DVE-3 complex, even with the neat monomers, the DBMR again does not appear to be as important. This result reflects the fact that the DMMA system is much slower to react than the MA complex as evidenced by the higher UV doses needed when compared with MA/ DVE-3 to achieve reaction especially in the neat 1.0/1.0 complexes. These conclusions are consistent for both PPE and cellulose.
3.5. Grafting of VA as donor with MA DVE-3 is a relatively expensive monomer and, if CT work is to be commercialised, lower cost donors would be an incentive. In this respect VA has potential
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Table 3 Significance of double bond molar ratio (DBMR) of DA components on photografting CT complexes in THF solventa CT complex
DBMR
MA/DVE-3
1.0/1.0
DMMA/DVE-3
1.0/1.0
MA/DVE-3
1.0/1.5
DMMA/DVE-3
1.0/1.5
a
Substrate
Solvent
No PI
PI
Dose (J)
Graft (%)
Dose (J)
Graft (%)
PPE Cellulose PPE Cellulose
0 0 THF THF
3 13 150 180
60 200 6 230
2 2 7 7
70 380 2 160
PPE Cellulose PPE Cellulose
0 0 THF THF
123 160 150 42
19 230 13 142
7 7 9 13
14 480 6 220
PPE Cellulose PPE Cellulose
0 0 THF THF
113 113 180 180
39 230 49 320
4 4 4 7
7 830 12 430
PPE Cellulose PPE Cellulose
0 0 THF THF
114 128 184 184
14 240 5 170
10 10 4 4
69 480 61 300
Monomer concentration (90% v/v); PI (if present), Irgacure 1800 (0.05% w/w); other conditions as in Table 1.
Table 4 Vinyl acetate as donor with acceptor maleic anhydride in photografting to PPE and cellulose in acetone solvent Substrate
PIa (%)
Monomer conc (%v/v) 80
PPE PPE Cellulose Cellulose a
1 0 1 0
60
40
20
Dose (J)
Graft (%)
Dose (J)
Graft (%)
Dose (J)
Graft (%)
Dose (J)
Graft (%)
37 220 37 220
8 0 26 6
110 220 110 220
14 0.1 5 6
220 220 110 220
5 1 1 8
220 220 110 220
9 0.4 3 6
Irgacure 1800 (1.0% w/w); DBMR, MA/VA (1.0/1.0); other conditions as in Table 1.
although the data in Table 4 for the photografting of MA/VA to both PPE and cellulose show that VA is much less reactive than DVE-3 in this work even with the incorporation of PI in the system. This situation is improved when DVE-3 replaces half the VA on a DBMR basis (Table 5), benzene being a particularly useful solvent in this respect with cellulose as substrate. The interpretation of these data is complicated by the fact that there are two donors with the one acceptor in the system, however the results show that VA has potential in this field.
Table 5 Vinyl acetate and DVE-3 as comonomer donors with acceptor maleic anhydride in photografting to cellulosea Solvent
Chloroform Toluene Benzene DMF THF
No PI
PI
Dose (J)
Graft (%)
Dose (J)
Graft (%)
24 24 24 24 24
4 0 266 5 0
3.7 3.7 3.7 3.7 3.7
0 13 11 43 0
a Monomer concentration (80% v/v); temperature 241C; DBMR, MA/VA/DVE-3 (1.0/0.5/0.5); other conditions as in
3.6. Role of gel pointFnature of copolymer As with any radiation grafting system, the role of homopolymer is important. In this respect the grafting of CT complexes is no exception. The CT system is
however more complicated than most conventional monomer radiation processes. In the former case, grafting in the presence of solvent is fastest at the highest monomer concentrations whereas with conven-
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L.-T. Ng et al. / Radiation Physics and Chemistry 62 (2001) 89–98 Table 6 UV conditions for photografting of MA/DVE-3 to cellulose without PI at gel point Monomer conc (%v/v) Solvent
Acetone CHCl3 CH2Cl2 THF
80
60 a
40 a
20 a
Dose (J)
Graft (%)
P
Dose (J)
Graft (%)
P
Dose (J)
Graft (%)
P
Dose (J)
Graft (%)
Pa
494 358 358 428
44 83 113 119
Ge Ge Ge Ge
494 428 358 494
19 68 51 53
N.Ge Ge Ge Ge
494 494 494 494
11 62 46 38
N.Ge Ge N.Ge N.Ge
494 494 494 494
8 66 17 8
N.Ge N.Ge N.Ge N.Ge
a P=polymer supernatant gelled (Ge); N.Ge=no gel; DBMR, MA/DVE-3 (1.0/1.0); temperature 231C; other conditions as in Table 1.
Table 7 UV conditions for photografting of MA/DVE-3 to cellulose with PI at gel point Monomer conc (%v/v) Solvent
Acetone CHCl3 CH2Cl2 THF a
80
60
40
20
Dose (J)
Graft (%)
Pa
Dose (J)
Graft (%)
Pa
Dose (J)
Graft (%)
Pa
Dose (J)
Graft (%)
Pa
5 5 5 5
42 43 52 396
Ge Ge Ge Ge
6 5 5 5
24 65 89 82
Ge Ge Ge Ge
6 5 5 6
51 27 75 31
Ge Ge Ge Ge
6 6 5 5
20 24 13 23
Ge Ge Ge N.Ge
Irgacure 1800 (0.05% w/w); other conditions as in Table 6.
tional monomer systems grafting tends to reach a peak at much lower monomer concentrations thus facilitating handling of the experimental system because of lower viscosity. More importantly, as the supernatant liquid in the CT system increases in viscosity due to homopolymer formation, the homopolymer formed tends to crosslink especially in the presence of bifunctional monomers like DVE-3 yielding a gel from which it can be difficult to extract the graft copolymer. For this reason in much of the current work the systems are taken almost to gel point, then the grafted substrate is removed. The data in Tables 6 and 7 illustrate this point. In Table 6 where MA/DVE-3 is photografted to cellulose without PI, the UV dose required to almost attain gel point in the supernatant varies considerably with the structure of solvent especially at the highest monomer concentrations studied. Consistent with earlier solvent experiments discussed in this grafting work, halogenated solvents are the most reactive and require the lowest UV doses to gel, this observation reflecting the ease with which radicals can be formed in these solvents upon exposure to UV. Inclusion of PI in these systems has a dramatic effect on the dose to gel (Table 7). Speed of polymerisation is now so fast, due to the additional radicals from the PI, that there is little difference in dose to gel amongst the various solvents, the dose being so low (5 J). Under the experimental conditions in Table 7, at the highest
monomer concentration studied in THF, the grafting yields are extremely high, a result which should be of value in preparative work. A further important aspect of the present CT photografting work is the nature of the graft copolymer formed. At high monomer concentrations, in appropriate CT systems, the grafting yield increases strongly as the polymerisation in the supernatant approaches gel point. Under these conditions, the polymer system tends to be severely crosslinked especially if multifunctional monomers are involved as one of the DA components. The difficulty of such a situation is that as the reaction reaches the gel point it can be difficult to remove graft copolymer. Care thus needs to be exercised in removing the grafted substrate prior to reaching gel. A further significant feature of the current work is that in addition to graft copolymers, the present system is of value in the synthesis of related IPNS and semi IPNS. The present authors are synthesising these materials from CT complexes. At this time one of the important problems in this grafting field is to characterise in detail the structures of the CT graft copolymers. Work on this problem is currently in progress, especially designed to overcome (i) crosslinking due to the presence of at least one binary functional component in the DA complex and (ii) the use of certain solvents which produce high radical yields that can complicate interpretation of the grafting process.
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3.7. Significance in curing The present data are relevant to analogous radiation curing processes. The type of monomers used in this CT monomer work are the same, or similar to, those used in UV curing. The present results illustrate the type of UV conditions where grafting can be expected to occur during the UV curing process. This observation is important in curing since the onset of concurrent grafting enhances the adhesion between the cured film and substrate and thus restricts the possibility of delamination occurring in the final product. This property is particularly relevant to naturally occurring macromolecules like cellulose and wool since, with these substrates in some applications, the presence of grafting during cure can render difficult the ultimate recycling of the finished product. In contrast this recycling problem with coated plastics like the polyolefins is not such a problem since these finished grafted products as waste materials can be relatively easily utilised elsewhere. Thus for some applications the presence of concurrent grafting in the final product is an advantage whereas in others it can be an impediment. The type of experimental UV conditions where such grafting may be observed are thus useful and are demonstrated in this paper. The present UV grafting work is also a guide to curing systems using ionising radiation like EB processes since previous work (Garnett et al., 1999b) has shown that UV and ionising radiation grafting systems are closely related. The radiation conditions required to achieve grafting established through UV can now be readily extrapolated to ionising radiation systems with the reverse process also possible.
4. Conclusions Polar solvents, especially the halogenated derivatives like DCM are shown to be effective in the photografting of a typical CT complex, MA/DVE-3, to representative trunk polymers like cellulose, wool and PPE. Aromatic solvents are also suitable for these processes even with polar substrates like cellulose and wool. Inclusion of appropriate PIs in the monomer solution enhances the grafting yield considerably, leading to lower UV doses to achieve a particular percentage graft. Photografting yields are highest at the highest monomer concentrations studied. Esterification of MA yields compounds which readily graft to cellulose as CT complexes but with lower efficiency than MA. The DBMR ratio of 1.0 : 1.0 is shown not to be critical for the photografting of complexes analogous to MA:DVE-3. VA is also a suitable donor in these processes with MA, although less reactive than DVE-3. Gel formation readily occurs with the more reactive CT complexes at relatively low UV doses. Mechanistically, the observed data are consistent
with the participation of a CT complex formed between the grafting monomers and active sites in the trunk polymer. The current photografting work is shown to be of value in the related fields of UV and EB curing.
Acknowledgements The authors thank AINSE and Ballina Pty Ltd for support.
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