Initiation of Polymerizations Involving Polysaccharides and Radio

17 Initiation of Polymerizations Involving Polysaccharides and Radio Frequency Cold Plasmas O. HINOJOSA, T. L. WARD, and R. R. BENERITO

Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch017

U.S. Department of Agriculture, Agricultural Research Service, Southern Regional Research Center, New Orleans, LA 70179

The ESR signals of cotton after exposure to both charged particles and uv light of rf cold plasma were identical but of greater intensity than those obtained when cotton was shielded from plasma particles by quartz or CaF . Chemiluminescence (CL) was oxygen dependent and greatest with unshielded cotton. CL of shielded and unshielded cottons was more prolonged than that of CL produced by irradiation with a Hg lamp. Plasma uv to 100 nm initiated graft polymerization of methacrylamide on cotton. A combination of depolymerization and polymerization was used with an equimolar N and H plasma to degrade chemically modified cottons in the electrode zone and to cause polymerization downstream to form products containing nitrogen and substituents of the chemically modified cottons. 2

2

2

The m o d i f i c a t i o n of cotton c e l l u l o s e by treatment with lowtemperature, low-pressure ammonia plasma created by passing ammonia gas through a radiofrequency ( r f ) e l e c t r i c f i e l d of 13.56 MHz has been reported (1). E a r l i e r reports (2,3,4) were on the e f f e c t s of r f plasma of argon, nitrogen or a i r on a group of polysaccharides that included cotton and p u r i f i e d c e l l u l o s e . In these e a r l i e r s t u d i e s , the polysaccharides were i n open sample holders within the r f r e a c t o r . Thus, samples were exposed to fast moving high temperature e l e c t r o n s , the slower moving p o s i t i v e and negative ions, and free r a d i c a l s as w e l l as to uv i r r a d i a t i o n s . In a l l types of r f plasmas i n v e s t i g a t e d , changes i n surface properties of the polysaccharides were analyzed by the techniques of e l e c t r o n spectroscopy for chemical analyses (ESCA), e l e c t r o n spin resonance (ESR), m u l t i p l e i n t e r n a l r e f l e c t a n c e i n f r a r e d spectroscopy (MIR) and chemiluminescence (CL). This chapter not subject to U.S. copyright. Published 1983, American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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Free r a d i c a l s i n d i c a t i v e of breakage of the g l u c o s i d i c bonds were detected as w e l l as e l e c t r o n i c a l l y e x c i t e d carbonyl groups in a l l plasma i r r a d i a t e d polysaccharides. The products of CL on exposure of these i r r a d i a t e d polysaccharides to oxygen can be a t t r i buted to r e l a x a t i o n of excited carbonyls and to t r a n s f e r of unpaired electrons to molecular oxygen. A l i n e a r regression analys i s of ESR and CL from a set of s i x saccharides of v a r i e d molecul a r s i z e and from another set of s i x c e l l u l o s e s from d i f f e r e n t plant o r i g i n s e s t a b l i s h e d that CL i n t e n s i t y i s h i g h l y c o r r e l a t e d with ESR i n t e n s i t y ( 5 ) . Thus, CL of an r f cold plasma i r r a d i a t e d polysaccharide can be predicted from the value of the ESR i n t e n s i ty of the i r r a d i a t e d sample. The use of r f c o l d plasmas to cause polymerization of monomers on such non-conducting substrates as g l a s s e s (6) and other polymeric substances (7) i s not new. However, the chemistry involved in r f c o l d plasmas i s complicated, and within the r e a c t o r , depolymerizations of substrates and of newly formed polymers occur simultaneously with polymerization r e a c t i o n s . C l a r k (8) has r e ported on such phenomena, and i t i s known that the r e l a t i v e rates of polymerization to depolymerization vary within the r e a c t o r . Depolymerization, p a r t i c u l a r l y of a polysaccharide substrate i s greatest between the electrodes and almost i n s i g n i f i c a n t downstream from the e l e c t r o d e s . Polymerization i s the dominant r e a c t i o n downstream from the e l e c t r o d e s , but within the glow area of the plasma. T h i s i s a report of a study to use depolymerization or degradation and polymerization reactions within the r f reactor to advantage i n the formation of new polymers and to i n v e s t i g a t e the r e l a t i v e c o n t r i b u t i o n s of r f produced p a r t i c l e s ( e l e c t r o n s and ions) and of r f produced uv l i g h t i n the i n i t i a t i o n of g r a f t p o l y merizations on cotton substrates. Cold Plasma Treatments D e t a i l s of the usual procedures followed with the 100-W, 13.56 MH r f generator and the c a p a c i t i v e l y and i n d u c t i v e l y coupled plasma reactor have been reported ( 2 ) . In Figure (1) i s a diagram of the reactor that i n d i c a t e s p o s i t i o n s of the e x t e r n a l e l e c t r o d e s , sample l o c a t i o n s , and p o s i t i o n of CaF2 sample holder used i n s p e c i f i e d experiments. In b r i e f , the reactor was evacuated to 20 mtorr before a selected gas used for the plasma was allowed to enter the reactor between the electrodes such that a flow rate of 0.1 standard cm^/sec was obtained. After s t a b i l i z a t i o n of the pressure i n the reactor to about 400 mtorr, the r f generator was turned on, the power adjusted to 40W, and the sample i r r a d i a t e d for the d e s i r e d time. A f t e r i r r a d i a t i o n , the reactor pressure was returned to atmospheric with Ar and samples were removed and stored i n Ar pending analyses. Z

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Analyses Chemiluminescence (CL) was measured on a l l samples immediately a f t e r plasma i r r a d i a t i o n and before exposure to a i r or moisture and again a f t e r exposure to room a i r . CL i n counts per min (CPM) was determined i n a Packard 3255 l i q u i d s c i n t i l l a t i o n spectromet e r equipped with low dark-noise p h o t o m u l t i p l i e r tubes (RCA 4501/V4) and a Parkard 585 l i n e a r recorder. E l e c t r o n spin resonance (ESR) s p e c t r a were obtained on a Varian 4502-15 spectrometer system equipped with a v a r i a b l e temperature accessory and a dual sample c a v i t y . E l e c t r o n spectroscopy f o r chemical analyses (ESCA) spectra were obtained on a Varian spectrometer Model VIEE-15 with a MgK x-ray source. Analyses were obtained on samples before and a f t e r plasma i r r a d i a t i o n s . Spectra represent e l e c t r o n binding energies ( E ) i n eV.


a

B E

Syntheses v i a Degradation and Polymerizations Others (9) have reported on the nature of s o l i d products a r i s ing from r f plasmas of C O a n d H2 that were e l e c t r o d e l e s s l y sustained at 13.56 MHz. While these i n v e s t i g a t o r s reported that h y d r o l y s i s of polymers showed presence of various amino a c i d s , i n c l u d i n g those with aromatic groups, such as t y r o s i n e , we (10) have formed proteinaceous m a t e r i a l l a c k i n g i n aromatic groups when the sole source of carbon w i t h i n the reactor was c e l l u l o s e . Aromatic groups were absent when the plasma gas was NH3 or combinations of N2 and H2 ( 5 ) . However, even with the same gaseous species i n a given plasma, the walls of the container and more importantly a s o l i d substrate such as a polysaccharide w i t h i n the r e a c t o r can have a chemical or c a t a l y t i c e f f e c t i n determining the f i n a l complex organic product v i a g a s - s o l i d phase processes. In attempts to form proteinaceous m a t e r i a l with aromatic subs t i t u e n t s , a cotton modified chemically to contain aromatic groups was placed i n the depolymerization zone, that i s , between the e x t e r n a l e l e c t r o d e s and an untreated cotton placed downstream i n the polymerization zone. The plasma was an equimolar mixture of N2 and H that had been shown to be optimal f o r the production of proteinaceous m a t e r i a l when cotton was placed between the e l e c trodes. According to ESCA analyses, the nitrogenous f i l m s formed downstream were i d e n t i c a l whether benzoylated c e l l u l o s e or benzyl a t e d c e l l u l o s e was placed between the e l e c t r o d e s . Figure (2) i l l u s t r a t e s the Ci$ spectra obtained with a cotton c o n t r o l (lowest curve) and with a cotton that had been treated i n a c o l d plasma of 1:1 mole r a t i o of N2 and H2 f o r one hour while a benzyl a t e d cotton placed between the e l e c t r o d e s acted as a source of carbon f o r syntheses of the proteinaceous product (uppermost c u r v e ) . The middle curve i s that f o r a cotton subjected to the 2 2 plasma i n the absence of the benzylated c o t t o n . The C^g 2

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PLASMA

REACTOR

SHIELDING (••PLATES WITH " 0 " RING I

(* Inlet 1

Electrode *


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Figure 2. Electron emission spectra for C electrons from cottons. Lowest curve is for an untreated cotton. Other curves are for cottons placed downstream in rf reactor with equimolar N :H plasma. For uppermost curve, a benzylated cotton was placed in the depolymerization zone between external electrodes during irradiation of cotton. ia

2

2


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peak at 284 eV with a f u l l width at h a l f height (FWHH) o f 3.0 eV i s t y p i c a l of c e l l u l o s e and other polysaccharides. The broadening of the C peak i n the product to a FWHH of 4.5 eV i s i n d i c a t i v e of formation of groups containing carbon atoms of higher and lower e l e c t r o n d e n s i t i e s than those i n the o r i g i n a l c e l l u l o s e . Aromatic groups contribute carbon atoms of E ^ about 284 and c a r bonyl groups contribute C atoms of Ε about 286 eV. No s i g n i f i cant d i f f e r e n c e s were detected when a benzoylated cotton was sub s t i t u t e d f o r a benzylated cotton as the source of aromatic groups. Use of phytic a c i d placed between the electrodes when the ^2 ^2 p l used r e s u l t e d i n nitrogenous product containing the phytate group. These are examples of the use of a s o l i d placed between the electrode as the source of a moiety to be used in a polymerization r e a c t i o n downstream from the e l e c t r o d e s . l s

E

ΰ


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a

s

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Photoinitiation A cotton p r i n t c l o t h was treated with an aqueous s o l u t i o n of 0.5M methacrylamide (MA), which has a c h a r a c t e r i s t i c absorption at 224 nm. I f such a f a b r i c sample i s a i r d r i e d before being enclosed i n a CaF£ c e l l and subjected to an argon plasma f o r 30 minutes, only a small amount of polymer i s g r a f t e d . A t y p i c a l N^g E S C A spectrum of a sample so treated i s the upper curve i n Figure (3). Attempts to do post polymerization of MA by f i r s t subjecting a cotton p r i n t c l o t h to 30 minutes of an Ar plasma and then immersing i t i n an aqueous s o l u t i o n of 0.5M MA also met with l i t t l e success. The lower curve i n Figure (3) i s the N^g spec trum of a sample i r r a d i a t e d i n an open container and then sub jected to MA. These ESCA spectra are the r e s u l t s of 100 scans. C r a f t i n g was s u c c e s s f u l when a cotton p r i n t c l o t h treated with aqueous 0.5M MA was enclosed while s t i l l wet i n the CaF2 sample holder and then i r r a d i a t e d i n an Ar plasma. The N-^g spectrum of a sample that had a 10% add-on i s shown i n the upper curve of Figure (4) and was obtained with only 10 scans. Note that spectra of Figure (3) are 100 scans. The c o n t r o l shown as the lower curve i n Figure (4) was obtained on a sample that had been treated with 0.5M MA f o r one hour, then washed and a i r d r i e d before being subjected to E S C A . The Ε of each peak i n Figures (3) and (4) i s 399.9 eV, i n d i c a t i n g amino type n i t r o g e n . The FWHH f o r the samples of highest add-on (upper curve F i g . 4) was the least and was 2.2 eV. The C^g E S C A spectrum f o r t h i s o p t i mally polymerized sample i s the upper curve of Figure ( 5 ) . The large FWHH o f 3.6 eV i s i n d i c a t i v e of s e v e r a l types of C]_g e l e c t r o n s . The lower curve of Figure (5) i s the C]_g spectrum of a cotton c o n t r o l showing a peak at 285 eV and a FWHH of 3.0 eV. Β

T

Ε

le

* ^1S peak c h a r a c t e r i s t i c of a carbonyl group i n polymerized MA would be about 287 eV and account f o r the s h i f t of the C^g spectrum to higher E g values. Other experiments have shown that MA can be g r a f t e d i f treated i n an open container within the E



·*

399 61M0IN6 ENERGY,


*

284 BINDING ENERGY, *V

274

1S

Figure 5. Electron emission spectra for C electrons from fabric of 10% add-on of methacrylamide described in Figure 4 (upper curve), and of an untreated cotton control (lower curve).

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r e a c t o r . Under such c o n d i t i o n s , the sample i s bombarded by e l e c trons, ions and r a d i c a l s as w e l l as by the short uv r a d i a t i o n s . When the sample i s enclosed i n a quartz container, which allows for passage of l i g h t as low as 2OU nm, some polymerization of MA i s obtained. The use of a CaF2 enclosure that passes l i g h t down to 100 nm allows f o r the photoinitiâtion of polymerization of MA.


ESR and

CL

The production of paramagnetic species i n c e l l u l o s e can be a t t r i b u t e d to i n t e r a c t i o n s between p a r t i c l e s , i n c l u d i n g e l e c t r o n s , ions, or free r a d i c a l s , with c e l l u l o s e or the e f f e c t s of uv i r r a d i a t i o n s on c e l l u l o s e . The ESR s i g n a l s obtained from cotton samples placed in open boats w i t h i n the r e a c t o r , within quartz sample holders, and within Cat^ sample holders showed that the s i g n a l obtained i n every case was s i m i l a r but d i f f e r e n t i n i n t e n s i t y . I f the i r r a d i a t e d samples are kept i n dry Ar, they are s t a b l e . In Figure (6) are the ESR s i g n a l s obtained a f t e r 1 week storage i n Ar of samples i r r a d i a t e d i n open containers (magnified IX), CaF2 container (magnified 2X), and quartz container (magnified 4X). These ESR spectra are f o r samples that had been i r r a d i a t e d for i d e n t i c a l times i n i d e n t i c a l zones of the plasma r e a c t o r , but with containers that provided for d i f f e r e n t s h i e l d i n g . The CL of the v a r i o u s l y i r r a d i a t e d samples were also i n v e s t i gated. If a cotton i s i r r a d i a t e d with the uv l i g h t from a mercury lamp f o r 30 minutes, the CL produced i s about 150,000 CPM. However, there i s an extremely f a s t decay and the CL i s i n s i g n i f i c a n t within 3-4 minutes. The decay i s f a s t even in dry N2 or Ar gases. Those samples subjected to plasma show a slower decay i n CL. Those samples subjected to Α Γ plasma i n open boats, quartz or CaF2 containers continued to chemiluiainesce even a f t e r a week. The CL values immediately a f t e r plasma treatments were extremely high and were reduced to about 10,000 - 15,000 CPM a f t e r a week, as shown by the i n i t i a l CPM i n Figure ( 7 ) . Figure (7) shows the increases i n uL with time of exposure of the sam ples to a i r . A l l three samples show increases i n CL on exposure. The increases for those samples s h i e l d e d by quartz and CaF2 are s i m i l a r . Greatest increase i n CL was observed with the sam ple treated i n the open boat. It should be noted that the open boat treated sample had the greatest ESR s i g n a l (Figure 6). Apparently, the f r e e r a d i c a l s produced by the f a s t moving e l e c trons or other p a r t i c l e s are more e f f e c t i v e i n producing CL i n the c e l l u l o s e than the free r a d i c a l s r e s u l t i n g from uv i r r a d i a t i o n at the d i f f e r e n t wavelengths.



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Figure 6. Electron spin resonance spectra for samples of cotton printcloth that had been treated for 30 min in an Ar plasma and then stored in Ar for 1 week. Sample in an open container (uppermost curve), sample shielded by CaF (middle curve), and sample shielded by quartz (lowest curve). Intensities are of 1,2, and 4 magnifications for comparative purposes. 2


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toor 9Ch


80h

Ό eoh

l υ

50h 40H

30

10

20

50 30 40 TIME (MINUTES)

60

70

Figure 7. Intensities of chemiluminescence in counts per minute after 1 week (t = 0) of storage of samples in Ar and with time (t) after exposure to air for samples irradiated in Ar plasma for 30 min in an open container (uppermost curve), a CaF container (middle curve), and a quartz container (lowest curve). 2


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Literature Cited 1. 2. 3. 4.


5. 6. 7. 8. 9. 10.

Ward, T.L. and Benerito, R. R. Textile Res. J., (1982), 52, 256-262. Jung, H. Z . , Ward, T. L. and Benerito, R. R. Textile Res. J., (1977), 47, 217-222. Ward, T. L., Jung, H. Z., Hinojosa, O. and Benerito, R. R., Appl. Polym. Sci., (1979), 23, 1987-2003. Ward, T. L., Jung, H. Z . , Hinojosa, O., and Benerito, R. R. J. Surface Sci., (1978), 76, 257-273. Ward, T. L. and Benerito, R. R. Polym. Photochem. (1982), Accepted for Publication. Goodman, J. J . Polym. Sci., (1960), 44, 551-552. Coleman, J . H., U.S. Pat. 3,600,122, Aug. 1971. Clark, D. Organic Coatings and Plastics Chem. (1980) 42, 460. Hollahan, J . R. and Emanuel, C. F. Biochimica et Biophysica Acta, (1970), 208, 317-327. Ward, T. L., Hinojosa, O., and Benerito, R. R. Organic Coatings and Applied Polymer Science Proceedings, (1981), 45, 382-385.

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Initiation of Polymerizations Involving Polysaccharides and Radio

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