Polymers for Fibers and Elastomers - American Chemical Society

18 Accelerated UV a n d R a d i a t i o n - I n d u c e d Grafting o f M o n o m e r s to Cellulose i n the Presence o f Additives Applications of These Copolymers in Art Restoration and Preservation

Downloaded by UNIV OF ARIZONA on April 23, 2017 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch018

CHYE H. ANG, JOHN L. GARNETT, STAN V. JANKIEWICZ, RON LEVOT, and MERVYN A. LONG Department of Chemistry, The University of New South Wales, Kensington, NSW 2033, Australia The value of ultraviolet light and ionizing radiation as initiators for the grafting of styrene in solvents to cellulose is discussed. Concepts common to both systems are compared. Thus the effect of structure of solvent and concentration of monomer on the efficiency of grafting are treated. The use of acid and polyfunctional monomers, also the synergistic effect of these two additives in enhancing grafting in both radiation systems is considered. The use of cellulose as a backbone polymer in these additive grafting studies is compared with analogous studies with wool, polyethylene, polypropylene and PVC. The significance of these additive effects in UV and EB radiation rapid cure (RRC) processes is discussed. Novel applications of both radiation and photografted celluloses in the field of art restoration and preservation are outlined. UV and ionizing radiation have been used extensively in the grafting of monomers to cellulose (1-10). Parameters affecting the yield of copolymerization and the efficiency of the process include structure of monomer, type of solvent used and the radiation dose and doserate. For preparative purposes, methods for increasing the grafting yields by the inclusion of additives are important. Most of the recent work using additives for enhancement purposes have been performed with synthetic backbone polymers (11-13) especially the polyolefins (11,12). Mineral acids have been utilized successfully as additives for increasing grafting yields in ionizing radiation initiated copolymerization of monomers to the polyolefins and PVC (13). Analogous acid effects have also been reported for the cellulose grafting system involving gamma ray initiation. By contrast with the ionizing radiation grafting studies, little additive enhancement work has been reported for UV initiated grafting reactions. It is the purpose of this paper to compare UV with ionizing radiation as initiators for grafting to cellulose, especially with respect to possible common parameters including additives which may determine optimum yields in both systems. The copolymerization 0097-6156/ 84/ 0260-0295S06.00/ 0 © 1984 American Chemical Society

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

296

POLYMERS FOR FIBERS AND ELASTOMERS


of styrene to c e l l u l o s e i s selected as model system. The use of acid as an additive for enhancing grafting with both UV and i o n i z i n g radiat i o n i s examined and the factors which determine the optimum y i e l d s of copolymerization with acid are considered. The acid e f f e c t as a general phenomenon i n UV and gamma ray grafting i s evaluated by comparing the cellulose data with analogous results from other representative trunk polymers including wool, the polyolefins and PVC. In addition to a c i d , new additives for enhancing copelymerization to the polyolefins are reported. The value of using these new additives with the c e l l u l o s e grafting system i s considered. The possible relevance of these grafting studies to the related f i e l d of UV and radiation rapid cure polymerization work i s examined. F i n a l l y novel applications of both radiation and photografted celluloses i n the f i e l d of art restoration and preservation w i l l be outlined. Experimental Styrene was supplied by Monsanto Chemicals (Aust.) Ltd. and was p u r i f i e d by column chromatography on alumina. The c e l l u l o s e used was Whatman 41 f i l t e r paper; wool was p l a i n weave f a b r i c made from 21 y diameter Merino wool f i b r e s ; polyethylene (low density) was supplied as f i l m (0.005") from Union Carbide; polypropylene was i s o t a c t i c , doubly oriented f i l m (0.002") ex-Shell; PVC f i l m (0.04") was commercially available GVan material. Grafting Procedures. The methods used f o r grafting were modifications of those previously reported (3,6,11,14). In the i o n i z i n g radiation work, grafting experiments were performed i n stoppered pyrex tubes (15 x 2.5 cm) containing styrene/solvent solutions (20 ml) at 20 ± 1 C. Backbone polymer films or s t r i p s of c e l l u l o s e of the appropriate size were f u l l y immersed i n the monomer solutions and the tubes i r r a d i a t e d i n a 1200 C i cobalt-60 source. At the comp l e t i o n of the g r a f t i n g , the films were removed from the solution, solvent washed and soxhlet extracted for 72 hours. With the acid solutions, e s p e c i a l l y for the c e l l u l o s e runs, the films and paper s t r i p s were washed with methanol:dioxan (1:1) p r i o r to soxhlet treatment. After removal of homopolymer, grafted polymers were dried to constant weight at 65% RH. In the UV work, monomer solutions (20 ml) were prepared i n stoppered pyrex tubes i n a manner s i m i l a r to that described previously for the gamma i r r a d i a t i o n system. The tubes were positioned on a motor driven ventilated c i r c u l a t i n g drum at distances of 12 to 30 cm from the UV source (90 W, high pressure, Hg type 93110E E , P h i l l i p s ) at 24 ± 1 C for the appropriate time as shown on the relevant tables. The polymer films were so positioned that, during i r r a d i a t i o n , the surfaces of the films were perpendicular to the incident radiation. After i r r a d i a t i o n , films and paper s t r i p s were treated as for the gamma system. 2

Results and Discussion Solvent Effects i n Radiation and Photografting of Styrene to Cellulose. The data i n Table I show that when the alcohols are used as solvents for the grafting of styrene to c e l l u l o s e , s i m i l a r trends i n r e a c t i v i t y are observed for both UV and i o n i z i n g radiation systems. Thus the



Comparison of UV with Ionizing Radiation for Grafting Styrene i n Alcohol Solvents to Cellulose

Styrene (% v/v)

a

D

Graft (%) Gamma Ray UV n-Butanol Methanol Methanol Ethanol Ethanol n-Propanol n-Propanol n-Butanol 20 0 20 14 30 13 5 5 9 82 0 12 28 40 43 67 17 5 0 34 22 18 60 51 105 5 51 62 132 0 30 50 5 80 15 53 J /l •a- v • -" . X 1A A Solutions contained uranyl n i t r a t e (1% w/v) as s e n s i t i z e r and i r r a d i a t e d f o r 24 hr at 24 cm from 90 W high pressure lamp

Table I.


1

I

I'

I f

aI-

o" s I

>

m H

o

>

298


lowest molecular weight alcohols, methanol and ethanol, give the highest grafting y i e l d s consistent with t h e i r wetting and swelling properties (5^-8). The UV results were obtained i n the presence of uranyl n i t r a t e as s e n s i t i z e r . Sensitized grafting i s much more e f f i c i e n t than copolymerization without s e n s i t i z e r . Under the UV conditions reported i n Table I grafting i s extremely low i f no uranyl n i t r a t e i s present. The solvent results i n Table I I are consistent with the above Table I I .


Styrene_ (% v/v)

Comparison of UV with Ionizing Radiation f o r Grafting Styrene i n Miscellaneous Solvents to Cellulose

D

UV Hex. DMF DMSO No Sens. Diox, (Diox.) 20 6 1 3 0 0 5 0 4 3 40 9 12 14 1 10 5 4 0 3 21 38 60 16 1 20 23 20 0 2 80 42 88 1 9 34 29 0 8 4 I r r a d i a t i o n i n stoppered vessel at 6.7 x 10^ rad/hr to t o t a l dose of 1.0 x 1 0 rad except hexane (7.7 x 1 0 rad/h). Where s e n s i t i z e r used, solutions contained uranyl n i t r a t e (1% w/v) and i r r a d i a t e d i n stoppered tubes f o r 24 hr at 24 cm from 90 W high pressure UV lamp, except f o r hexane (benzoin ethyl ether, 1% w/v). a

Gamma Ray Diox. DMF DMSO

Hex.

6

4

alcohol data. Those solvents such as dimethyl formamide, dimethyl sulfoxide and dioxan which wet and swell the cellulose also give r e l a t i v e l y high grafting y i e l d s with both radiation i n i t i a t o r s . Grafting i n DMSO i s p a r t i c u l a r l y e f f i c i e n t at high monomer concentrations using sensitized UV. Grafting y i e l d s i n the absence of uranyl n i t r a t e are very low (< 4%). Although wetting and swelling are important with this group of solvents, i t i s known that t h e i r swelling characterist i c s are s i g n i f i c a n t l y different to those of the alcohols. Thus i n comparing the data from the two groups of solvents, i t i s obvious, p a r t i c u l a r l y with the alcohols, that chemical processes also part i c i p a t e i n grafting, presumably involving r a d i c a l reactions from the r a d i o l y s i s of the alcohol as previously proposed (6)• Mechanistically the processes involved i n grafting styrene i n a polar solvent to cellulose with i o n i z i n g radiation and sensitized UV are analogous. Each process i s predominantly free r a d i c a l i n nature with a possible contribution from energy transfer processes. In the gamma ray system grafting s i t e s i n the c e l l u l o s e (CelloH) are formed by the sequence of processes shown i n Equations 1 to 3 with methanol as representative solvent. CelloH

—>

CH 0H 3

CelloH + CH.O*

Cello* + H*

(1)

CH 0*

(2)

3

-y

+ H*

CH-OH + Cello*

At temperatures of the copolymerization, p a r t i c i p a t i o n of i o n i c process i s minimal.


(3)

18.

UV & Radiation-Induced

ANG ET AL.

Grafting of Monomers

299

With the uranyl n i t r a t e sensitized UV system, r a d i c a l s i t e s i n the c e l l u l o s e are formed from reaction with excited uranyl i o n species (Equations 4-6). + hv

2+

V0

+]

[V0* *

+

— *

CelloH

[U(>2 ] * +

Cello'

1

[uo^""]* + CH3OH

-*> C H O ' 3

(4) + H

+

+ H

+

U O ^

(5) +

+

+ uoi;

(6)


Thus i n both radiation systems, monomer can d i f f u s e to a s i t e and graft by the charge-transfer mechanism previously proposed (3,5,15). E f f e c t of Acid as an Additive on UV and Radiation Grafting. Previous studies have shown that mineral acids can enhance the radiation g r a f t i n g of styrene to c e l l u l o s e under certain radiation conditions. (6). The r e s u l t s of analogous experiments i n photografting are shown i n Table I I I , and compared with the corresponding gamma ray data. In Table I I I .

Comparison of UV and Ionizing Radiation f o r Acid Enhancement E f f e c t s i n Grafting Styrene i n Methanol to Cellulose

Graft (%) Styrene Gamma Ray UV (% v/v) H P0 N.A. H S0L HN0« H.PO, N.A. H S07 HClHN0 4 (TM) (2M) (0.02M) (0.67M) 10 6 28 15 27 7 9 9 b 54 20 30 54 34 20 83 29 26 13 b 68 20 40 39 28 73 47 23 31 29 b 72 37 60 47 32 91 23 30 34 77 b 92 21 77 56 80 32 95 127 53 29 b 90 167 116 131 107 Dose rate of 2.64 x 10^ rad/hr to t o t a l dose of 2.0 x 10^ rad i n stoppered tubes Phase separation occurred Acid (1% v/v) with uranyl n i t r a t e as s e n s i t i z e r (1% w/v) and i r r a d i a t e d f o r 24 hr at 24 cm from 90 W high pressure UV lamp. a

9

H

C

C

1

3

2

3

4

-

a

-

-

-

-

-

D

c

both radiation systems i t i s observed that a l l mineral acids studied increase the g r a f t i n g y i e l d s at l e a s t at one p a r t i c u l a r monomer concentration. S u l f u r i c and hydrochloric acids are the most e f f i c i e n t enhancement acids with gamma ray grafting whereas a l l acids, part i c u l a r l y phosphoric, lead to very s i g n i f i c a n t increases i n copolymerization y i e l d s with UV. In the i o n i z i n g radiation system, with i n c l u s i o n of a c i d , a peak i n grafting occurs at low monomer concentrations (20% with s u l f u r i c and 10% with hydrochloric a c i d ) . Add i t i o n of acid also r e s u l t s i n g r a f t i n g enhancement at monomer concentrations removed from the peak (10% with s u l f u r i c , n i t r i c and phosphoric a c i d s ) . Analogous r e s u l t s are also observed i n the UV system, except that the peak or Trommsdorff e f f e c t (6)in copolymerization s h i f t s to high monomer concentrations (90%). These acid enhancement e f f e c t s are also dependent on radiation dose as demonstrated by the data i n Figure 1, the s p e c i f i c g r a f t i n g y i e l d s at the various doses being shown i n Table IV. With increasing dose, there i s almost an exponential increase i n enhancement with grafting at


300


Table IV.

Effect of Acid as the Dose i s Increased on the Radiation Grafting of Styrene i n Methanol to C e l l u l o s e . 3

Total Dose Graft (%) , (x 10 rad) Neutral H S0, (1.0 x 10~ M) 10 1.0 7 2.0 35 26 136 5.0 101 7.0 208 121 321 8.0 151 10.0 514 219 Styrene (30% v/v) i n methanol grafted at 3.0 x 10^ rad/hr at 15°C 5

o

4


a

constant s u l f u r i c acid addition. Grafting can also be performed i n a i r with almost the same e f f i c i e n c y as i n evacuated vessels, the gamma ray system being s i m i l a r to UV grafting (Figure 2) i n this respect. Mechanistically the role of acid i n enhancing gamma radiation copolymerization involves a number of competing processes. In previous work i n t h i s f i e l d with cellulose (6), i t has been suggested that acid at the concentrations used does not markedly a f f e c t the p r e c i p i t a t i o n of the grafted chains or the swelling of the backbone polymer, e s p e c i a l l y with c e l l u l o s e . Instead i t has been proposed that part of the acid enhancement can be attributed to a radiation chemistry phenomenon involving increased G(H) y i e l d s and thermalized electrons (Equations 7 to 9). These processes can lead +

CH 0H

+H —*

3

CH 0H 3

CelloH

+ 2

CH OH 3

+

(7)

2

+ e —CH 0H + H

(8)

3

+

+ H — * CelloH

+

(9)

2

to increased numbers of grafting s i t e s by abstraction reactions with the trunk polymer (Equation 10). More recent work on the acid e f f e c t CelloH + H

— * Cello* + H

2

(10)

indicates that acid leads to an increase i n styrene-solvent i n t e r mediates (MS*) i n the grafting solution, such species r e s u l t i n g i n more grafting s i t e s (Equation 11). In these l a t e r studies (16), i t MS*

+

CelloH

* MSH

+

Cello*

(11)

has also been observed that i n the presence of acid, the chain length of oligomer i n the grafting solution i s shortened and the numbers of these shorter chains are increased. Shorter oligomer chains could diffuse more readily into the swollen backbone polymer to achieve more e f f i c i e n t termination at a grafting s i t e . The increase i n concentration of oligomer chains i n the bulk of the solution would also result i n an increase i n the v i s c o s i t y of both the grafting solution and the solution that i s absorbed within the backbone swollen polymer, thus leading to the enhanced Trommsdorff peak as observed experiment a l l y i n the presence of acid.


18.

ANG ET AL.

UV & Radiation-Induced Grafting of Monomers

301

600

O


•

0.2

0.4 0.6 Dose (Mrad)

0.8

0.1 M H So ; 2

4

Neutral

1.0

Figure 1. Acid enhancement i n g r a f t i n g of styrene to c e l l u l o s e . Dose rate 0.03 Mrad h r " . Styrene 30% (v/v) i n methanol. 1

80 r

70

60

50

#

vacuum;

A

air

40

30

20

10

0

10

20

30

AO 50 60 70 Styrene (V.)

80

90

Figure 2. Grafting y i e l d of styrene i n methanol, i n vacuum and i n a i r f o r c e l l u l o s e UV system. S e n s i t i z e r conditions (Table I ) .



302

The range of differences depicted i n Figure 1 for the component affect grafting i n a number of ways. Thus the presence of residual water i n c e l l u l o s e 7% at ambient conditions) can lead to acid enhancement i n photosensitized grafting by processes s i m i l a r to the s u l f u r i c acid catalyzed photopolymerization of monomers i n aqueous solution where, i n the presence of acid (e.g. H^SO^), additional radicals are formed. These may subsequently react with trunk polymer by abstraction reactions to give increased numbers of grafting s i t e s (Equations 12,13). Changes i n the physical parameters of cellulose 2

S0 " 4


S0

T 4

+

H0

+

H 0

2

+

3

T

*>

S0

>

H S0

+ H" + OH"

4

2

(12)

-

4

+ OH

(13)

by the i n c l u s i o n of acid can also advantageously a f f e c t g r a f t i n g . Thus acid can ( i ) produce both i n t e r c r y s t a l l i n e and i n t r a c r y s t a l l i n e swelling which loosens the "order-disorder" structure of the c e l lulose making i t more accessible to reagents and ( i i ) act as a catalyst i n the hydrolysis of the c e l l u l o s e leading to uncoiling of the chains and improved monomer a c c e s s i b i l i t y . Such hydrolysis r e actions involve an intermediate complex between the g l y c o s i d i c oxygen and the proton. In the presence of UV, breakage of the glycosidic bond of the complex i s f a c i l i t a t e d leading to additional grafting s i t e s . Comparison of Cellulose with Wool, the Polyolefins and PVC i n Acid Enhanced Grafting. The results i n Table V show that acid enhancement i n g r a f t i n g i s a general phenomenon which i s applicable to a range of Table V.

Comparison of Cellulose with Other Backbone Polymers (Wool, Polyethylene, Polypropylene and PVC) f o r Acid Enhanced Radiation Grafting of Styrene i n Methanol

Styrene (% v/v)

Cell.* N.A. IT" 20 8 24 30 17 33 40 22 31 50 21 27 60 28 25 Dose rate of 4.0 M H S0 Dose rate of 2.5 M H S0 Dose rate of 4.0 M H S0 Dose rate of 4.0 M H S0, Dose rate of 1.0 M H S0 1

a b c d e

2

Graft (%) Polyeth. Polyprop. PVC H+ N.A. H N.A. N.A. H+ 0 10 29 218 57 63 34 94 150 13 75 130 66 70 27 58 50 85 70 100 25 6 68 75 37 55 10 36 45 3 28 60 83 73 x 104 rad/hr to 2.0 x 10^ rad with 4.0 x 10- -1 WoolP H+ N.A. 8 38

-

c

d

e

+

-

4

2

4

2

4

4

x 10

rad with 2.0

x 10"

4

x 10"* rad with 2.0

x 10"

4

x 10"* rad with 2.0

x 10"

5

x 10

x

10 rad/hr to 2.0

x

10 rad/hr to 2.0

x

10 rad/hr to 3.0

x

10 rad/hr to 1.0

5

o

2

5

rad with 7.5

x 10"

-1 -2 -1 -2

4

backbone polymers of widely d i f f e r i n g structures. One of the s i g n i f i c a n t features of the data i n the table i s the d i f f e r i n g radiation conditions required to observe the Trommsdorff peak for each of the polymer systems studied. Thus f o r PVC, the gel e f f e c t occurs at 40%



18.

ANG ET AL.


303

monomer concentration i n both neutral and acid solutions whereas with polypropylene, using the same monomer/solvent system, the peak i s observed at 30% styrene i n neutral solution and at 20% i n a c i d i f i e d methanol. With polyethylene, the Trommsdorff e f f e c t occurs at 40% monomer i n neutral solution and 30% i n acid, whilst with wool, no g e l e f f e c t i s seen i n either neutral or acid solution. For a l l backbone polymers, the grafting y i e l d s , under the radiation conditions used, are of comparable orders of magnitude except for PVC where the higher y i e l d s may r e f l e c t the fact that during i r r a d i a t i o n PVC generates HCl i n s i t u which may act as acid additive and accelerate grafting. The acid results with wool indicate the importance of dose-rate e f f e c t s i n grafting. At much lower dose-rates, i n the presence of acid, wool grafts to extremely high y i e l d s (17), i n d i c a t i n g that the basic wool structure i s susceptible to bond rupture at low radiation doses thus increasing the s i t e s available for grafting i n t h i s trunk polymer. The ease of grafting wool i s exemplified i n the UV system (Table VI) where yields of over 2400% are observed i n the presence of acid. In the UV experiments, the u l t r a v i o l e t l i g h t i s delivered over 24 hours and t h i s r e l a t i v e l y slow radiation dose-rate accentuates the y i e l d of graft with wool. The functional groups i n wool also absorb UV strongly and assist the sensitized grafting, thus copolymerization y i e l d s i n the presence of acid without s e n s i t i z e r are an order of magnitude higher than when s e n s i t i z e r alone i s present. With both s e n s i t i z e r and acid, y i e l d s are an order of magnitude higher again. Other authors have discussed UV grafting to wool (18-20) but have not reported the use of the s e n s i t i z e r s discussed here and have not examined acid e f f e c t s . Their results confirm that UV grafting to wool i s a f a c i l e reaction. With respect to the other backbone polymers i n UV grafting, PVC i s as expected whereas the results for the p o l y o l e f i n s , e s p e c i a l l y polyethylene, are surprising when compared with c e l l u l o s e . The r e l a t i v e l y high copolymerization y i e l d s with polyethylene may be attributed to the presence of trace impurities i n the backbone polymer which are present during synthesis. These impurities s e n s i t i z e the grafting process. Comparison of Acid with Polyfunctional Monomers as Additives i n Polyethylene Grafting. New additives, as well as acid, have been found to accelerate grafting i n the presence of i o n i z i n g radiation. Much of this recent work has been performed with polyethylene. Only very preliminary studies of these new additives have been carried out with c e l l u l o s e , however these early r e s u l t s suggest that these l a t e s t additives w i l l also be very valuable f o r copolymerization reactions with c e l l u l o s e . The more comprehensive polyethylene data w i l l therefore be presented here to act as a guide for what might be predicted when cellulose i s used as backbone polymer. The new additives are polyfunctional monomers, which are exemplified by divinylbenzene (DVB) and trimethylol propane t r i acrylate (TMPTA) and are used i n concentrations of 1% (v/v). When these polyfunctional monomers are added to the monomer solution, the results show that the gamma ray grafting of styrene i n methanol to polyethylene i s enhanced to a comparable degree to that of i n c l u s i o n of acid (Table VII). The polyfunctionality of DVB and TMPTA i s obviously important i n these reactions, since the number of grafting s i t e s i n the system i s p o t e n t i a l l y enhanced by the increase i n



13 28 34 53

54 73 77 95

Cellulose N.A. BT

3

N.A. N.S. S. 2 23 22 29 55 17 53 40

0

H+ N.S. S. 8 160 170 246 1506 285 2487

Wool 0

Graft (%) Polyethylene N.A. N.S. S. N.S. 0 26 18 2 48 149 10 24 41 8 18 15 S. 21 132 39 22

0

Polypropylene H+ N.A. N.S. N.S. S. 0 17 5 38 1 163 30 6 44 6 26 13

S. 66 166 37 15

PVC N.A. S. 18 67 16 15 —

a

—

+

H S. 26 44 19 11

+

Uranyl n i t r a t e (1% w/v) as s e n s i t i z e r . Irradiations at 30 cm f o r 24 hr from 90 W high pressure UV lamp. S u l f u r i c acid (1% w/v). Benzoin ethyl ether (1% w/v)sensitizer. Irradiated as i n footnote b. Benzoin ethyl ether (1% w/v) s e n s i t i z e r , Irradiated as i n footnote b. At 30% monomer concentration, graft as follows (N.A. = 43, H = 77).

20 40 60 80

Styrene (% v/v)

Table VI. Comparison of Cellulose and Other Backbone Polymers (Wool, Polyethylene, Polypropylene and PVC) f o r Acid Enhanced UV Grafting of Styrene i n Methanol


t-n r > (/> H O w

o

>

Vi

73

m

DO

73

3

in

m 73

S

18.

ANG ET AL.

Table VII.

305


Comparison of Acid with Polyfunctional Monomers as Additives f o r Enhancing UV and Ionizing Radiation Grafting of Styrene i n Methanol to Polyethylene Film

Graft (%) UV Gamma Raya DVB Neutral H H+ TMPTA DVB TMPTA Neutral 20 14 28 14 58 19 15 28 30 52 37 74 126 51 41 101 39 321 40 76 81 193 203 74 189 73 412 50 385 109 134 107 136 124 137 322 60 264 89 121 119 70 133 89 73 37 31 191 105 80 68 62 32 77 39 74 59 Dose rate_of 4.1 x 10^ rad/hr to 2.4 x 10^ rad with s u l f u r i c acid (2.0 x 10 M) , divinylbenzene (DVB) and trimethylol propane t r i a c r y l a t e (TMPTA) at 1% (v/v) Irradiated at 24 cm f o r 24 hr from 90 W high pressure UV lamp. Other conditions as i n footnote a. Benzoin ethyl ether (1% w/v).

Styrene (% v/v)

b

+

-


-

-

-

-

-

1

unsaturation due to the presence of the type of additive. Thus with polyfunctional monomers such as DVB i n additive amounts, branching of the grafted polystyrene can occur when one end of the DVB, immobil i z e d during grafting, i s bonded to the growing chain. The other end i s unsaturated and free to i n i t i a t e a new chain growth v i a scavenging reactions. The new branched polystyrene chain may eventually t e r minate, cross-linked by reacting with an adjacent polystyrene chain or immobilized DVB r a d i c a l . Grafting i s thus enhanced mainly through the branching of the grafted polystyrene chain. The results from the addition of TMPTA can be explained i n an analogous manner. In the UV i n i t i a t e d grafting reactions, the addition of DVB and TMPTA accelerate the process s i m i l a r l y to that observed i n the gamma ray system. However the magnitude of the enhancement with the polyfunctional monomers with UV i s much higher than with acid. The presence of these monomers also causes a s h i f t i n the Trommsdorff peak from 40 to 50% styrene concentration. The mechanism of the DVB and TMPTA enhancement i n grafting with UV appears to be the same as for the gamma system. Synergistic E f f e c t of Acid and Polyfunctional Monomers as Additives i n Polyethylene Grafting. As with the previous section, data i n this section for grafting to polyethylene w i l l be presented as a guide f o r what may be expected generally i n copolymerization, especially f o r c e l lulose. Thus i t i s found that i f acid and the polyfunctional monomers are included i n the same grafting solution, a synergistic e f f e c t of the two additives i s observed. For the gamma ray grafting of styrene i n methanol to low density polyethylene, the e f f e c t s of DVB and TMPTA are very much enhanced i n the presence of acid (Table VIII). As previously discussed, the i n c l u s i o n of acid increases the concentration of H* and MS" species (Equations 8 and 11) i n the grafting solution. This increase i n r a d i c a l concentration when combined with increased unsaturation i n the grafted chains due to the polyfunctional monomers leads to branched polystyrene chains of higher molecular weight and hence higher g r a f t . An analogous explanation can be proposed for the synergistic e f f e c t of acid and polyfunctional monomer enhancement i n grafting i n the UV system (Table VIII).


306


Table VIII.

Synergistic E f f e c t of Acid and Polyfunctional Monomers as Additives for Enhancing UV and Ionizing Radiation Grafting of Styrene i n Methanol to Polyethylene Film

Styrene (% v/v)

Graft (%) UV^ Gamma Ray Neutral DVB+H+ DVB+H"*" TMPTA+H"*" TMPTA+H+ Neutral 20 14 41 43 27 28 30 37 78 98 58 54 101 40 266 76 230 119 106 189 50 446 525 109 188 124 181 60 89 298 371 156 70 188 89 112 187 37 101 80 68 61 32 89 95 Dose rate of 4.1 x 10^ rad/hr to 2.4 x 10^ rad with s u l f u r i c acid (2.0 x 10" M), DVB and TMPTA at 1% (v/v). Irradiated at 24 cm f o r 24 hr from 90 W high pressure UV lamp. Other conditions as i n footnote a. Benzoin ethyl ether (1% w/v). a

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Significance of Additive Enhancement Effects i n Grafting. The use of additives such as acids and the polyfunctional monomers to enhance radiation and photografting of monomers to backbone polymers i s important i n a preparative context since any procedure for increasing y i e l d s at constant dose w i l l be extremely valuable f o r those trunk polymers which are susceptible to degradation by i o n i z i n g radiation or UV. Thus, the lower the dose to achieve a p a r t i c u l a r graft, the more chemically stable the f i n a l copolymer should be. The fact that both acid and polyfunctional monomers can be used f o r t h i s purpose i s also advantageous, since each type of additive may give a copolymer possessing a d i f f e r e n t structure. With the polyfunctional monomers some cross-linking may be expected whereas, with acid alone, the l e v e l of cross-linking should be minimal. F i n a l l y when acid i s used as an additive i n these g r a f t i n g reactions, p a r t i c u l a r l y with c e l l u l o s e , care should be exercised i n the solvent extraction procedure f o r homopolymer removal. Thus when the crude graft copolymer d i r e c t l y from i r r a d i a t i o n i s being treated at elevated temperature i n a soxhlet apparatus, any residual acid remaining i n the c e l l u l o s e w i l l be concentrated and w i l l subsequently attack the backbone polymer leading to severe acid degradation. Cellulose i s p a r t i c u l a r l y prone to t h i s reaction e s p e c i a l l y when strong oxidizing acids such as n i t r i c are present. I t i s therefore recommended that when acid additives are used, the backbone polymer be thoroughly washed with the appropriate cold solvent (methanol for cellulose) to remove a l l acid p r i o r to extraction. Value of these Copolymers i n Art Restoration and Preservation. Currently, the present authors are involved i n a unique application of radiation polymerization techniques where the present grafting work can be of value. Thus radiation polymerization i s becoming i n creasingly useful i n the restoration and preservation of art treasures (21, 22), p a r t i c u l a r l y for materials of c e l l u l o s i c o r i g i n . Methods now used include impregnation of c e l l u l o s i c s such as wood with acrylate monomers followed by i r r a d i a t i o n . More recent



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ANG ET AL.

UV & Radiation-Induced

Grafting of Monomers

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developments involve radiation rapid cure (RRC) treatments where surface modification of c e l l u l o s i c s , wool and leather are important. In the RRC techniques both electron beam and UV are i n i t i a t i n g sources and the polymer system includes a low molecular weight oligomer d i s solved i n an appropriate monomer. The results of the present additive e f f e c t studies i n radiation grafting are of significance i n the above restoration work since, i n both impregnation and RRC procedures e s p e c i a l l y with c e l l u l o s e , wool and leather materials, there i s the p o s s i b i l i t y that simultaneous grafting occurs with the polymer formed from the oligomer mix (23) . Tritium l a b e l l i n g studies show that grafting can be appreciable i n such situations (23). The fact that additives are a v a i l a b l e , esp e c i a l l y acid type materials, which can protonate the OH groups of c e l l u l o s i c s to form potential s i t e s f o r grafting i s also important, since, by this mechanism, y i e l d s of graft can be increased. The occurrence of grafting, during RRC treatments p a r t i c u l a r l y , i s an advantage since properties such as adhesion, f l e x i b i l i t y and toughness of the finished material can be s i g n i f i c a n t l y improved. In a unique application of RRC, the present authors are preserving timber and leather artefacts from Henry V I I I s warship, The Mary Rose, which sunk i n the Solent and was recently recovered a f t e r 438 years on the seabed. The fact that, using the additives outlined i n t h i s paper, grafting may be increased during these RRC treatments i s very important, since the enhancement e f f e c t may well improve the physical properties of the material and thus should a s s i s t the longevity of the f i n a l product of the preservation process. 1

Acknowledgments The authors thank the Australian I n s t i t u t e of Nuclear Science and Engineering, the Australian Research Grants Committee and the Australian Atomic Energy Commission f o r f i n a n c i a l assistance.

Literature Cited 1. 2. 3.

Krassig, H.A.; Stannett, V.T. Advan. Polym. Sci. 1965, 4, 111. Arthur, J.C. Jr. Advan. Chem. Ser. 1971, 99, 321. Dilli, S.; Garnett, J . L . ; Martin, E.C.; Phuoc, D.H. J. Polym. Sci. C 1972, 37, 57. 4. Nakamura, Y.; Schimada, M. In "Cellulose Chemistry and Technology; Arthur, J.C. Jr. Ed.; ACS SYMPOSIUM SERIES No. 48, American Chemical Society: Washington, D.C., 1977; p.298. 5. Hebeish, A.; Guthrie, H.T. "The Chemistry and Technology of Cellulosic Copolymers", Springer-Verlag, Berlin Heidelberg, 1981. 6. Garnett, H.L. In "Cellulose Chemistry and Technology"; Arthur, J.C., J r . , Ed.; ACS SYMPOSIUM SERIES No. 48, American Chemical Society: Washington, D.C., 1977; p.334. 7. Geacintov, N.; Stannett, V.T.; Abrahamson, E.W.; Hermans, J . J . J. Appl. Polym. Sci. 1960, 3, 54. 8. Arthur, J.C. Jr. Polym. Preprints 1975, 16, 419. 9. Kubota, H.; Murata, Y.; Ogiwara, Y. J. Polym. Sci. 1973, 11, 485. 10. Tazuke, S.; Kimura, H. Polym. Lett. 1978, 16, 497. 11. Ang, C.H.; Garnett, J . L . ; Levot, R.; Long, M.A. J. Polym. Sci. Polym. Lett. 1983, 21, 257. 12. Chappas, W.J.; Silverman , J. Rad. Phys. Chem. 1979, 14, 847.


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Barker, H.; Garnett, J.L.; Levot, R.; Long, M.A. J. Macromol. Sci-Chem. 1978, A12(2), 261. Garnett, J.L. Rad. Phys. Chem. 1979, 14, 79. Dilli, S.; Garnett, J.L. J. Polym. Sci. 1966, A - l , 2323. Ang, C H . ; Garnett, J . L . ; Jankiewicz, S.J.; Sangster, C. In "Graft Polymerization of Lignocellulosic Fibers;" Hon, D.N.S., Ed.; ACS SYMPOSIUM SERIES No. 187, American Chemical Society: Washington, D.C., 1982, p. 141. Garnett, J . L . ; Leeder, J.D. In "Textile and Paper Chemistry and Technology;" Arthur, J.C. J r . , Ed.; ACS SYMPOSIUM SERIES No. 49, American Chemical Society: Washington, D.C., 1977, p. 197. Needles, H.L.; Wasley, W.L. Text. Res. J. 1969, 39, 97. Bamford, C H . ; Crowe, P.A.; Wayne, R.P. Proc. Roy. Soc., Ser. A. 1965, 284, 455. Ishibashi, H.; Oku, M. Cirtel III 1965, 385. Garnett, J . L . ; Major, G. Inst. Conserv. Cultural Material Bull. 1979, S(2), 49. Davis, N.P.; Garnett, J . L . ; Long, M.A.; Major, G.; Nicol, K.J. In "Preservation of Paper and Textiles of Historic and Artistic Value II;" Williams, J . C . , Ed.; ADVANCES IN CHEMISTRY SERIES No. 193, American Chemical Society: Washington, D.C., 1981; p.223. Ang, C.H.; Davis, N.P.; Garnett, J.L.; Yen, N.T. Rad. Phys. Chem. 1977 9(4-6), 831.

RECEIVED May 15, 1984


Polymers for Fibers and Elastomers - American Chemical Society

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