Some Physical Properties and Structure Determination of Vinyl

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Ind. Eng. Chem. Res. 2009, 48, 9338–9345

Some Physical Properties and Structure Determination of Vinyl Monomer-Grafted Antheraea assama Silk Fiber A. M. Das,* P. K. Chowdhury, C. N. Saikia, and P. G. Rao North East Institute of Science & Technology, Jorhat - 785 006, Assam, India

Vinyl monomer was used in aqueous medium with CeIV initiator for chemical modification of Antheraea assama silk fiber at different reaction conditions. The rate of grafting (%) was evaluated as a function of the concentration of monomer (AAm) and initiator (CeIV). The graft copolymerization of silk-fibroin with vinyl monomer (acryl amide) were characterized with Fourier transform infrared spectroscopy (FT-IR) and UV-vis spectroscopy (UV). Thermal analysis was done by thermogravimetric (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) techniques. The kinetic parameters were also studied using Coats and Redfern method with Fortran 77 computer programming. The structural changes of the silk molecule, grafted with vinyl monomer, were studied in relation to the weight gain and X-ray diffraction curves. Moreover, the grafted products were further evaluated to see the improvement in water staining, water retention capacity, and also tensile properties. Introduction Silk fibroin is a well-described natural fiber produced by the silkworm, Antheraea assama Westwood (Lepidoptera: Saturniidae), a multivoltine, sericogenic insect native to North Eastern India1 which has been used traditionally in the form of thread in textiles for thousands of years. The silk contains a fibrous protein that forms the thread core and a gluelike protein termed as sericin that surrounds the fibroin fibers and cements them together.2 Recent interest lies in the application of silk fibroin (SF) as a biomaterial because of the unique mechanical properties of silk fibers as well as its biocompatibility and biodegradability.3,4 Natural silk fibers are one of the most beautiful and precious fibers, possessing excellent handling and outstanding mechanical and esthetic properties due to which they are exploited for production of precious textile goods. However, it may be possible to resolve some issues by blending the silk fibroin with other natural or synthetic polymers, such as poly(vinyl alcohol),5 sodium alginate,6 cellulose,7 chitosan,8 etc. The use natural protein fibers/fabrics in graft copolymerization has become an attractive means of chemical modification of these fibers/fabrics since such treatment, in general, improves some of the disadvantages associated with them. Recently, graft copolymerization has become one of the finest methods for modifying the natural silk fiber through the creation of branches of synthetic polymers, which impart certain desirable properties to the fiber without destroying its basic properties. However, silk lacks some important performance properties, such as wrinkle recovery, wash and wear ability, photo yellowing, thermal stability, and abrasion resistance, and also contains water staining characteristics which seriously impede general use of silk fiber/fabric, which can be improved by graft copolymerization and/or other chemical modification techniques.9-12 Qualitative and quantitative analysis of methyl methacrylate (MMA) and acryl amide (AAm) grafted silk fiber exhibited improved performance properties.13 In this manuscript we have improved these disadvantageous properties by graft copolymerization technique using vinyl monomer (acryl amide) and CeIV initiator system. The graft copolymerization of silk fibroin with viny monomer (acryl * To whom correspondence should be addressed. E-mail: [email protected].

amide) was characterized with Fourier transform infrared spectroscopy (FT-IR) and UV-vis spectroscopy (UV). Thermal analysis was done by thermogravimetric (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) techniques. Moreover, the grafted silk fibroin with acrylamide (SH-AAm) polymer was further evaluated to highlight the improvement in water staining, water retention capacity, and also on tensile properties. Thus, the successful preparation of silk fibroin with AAm monomer by graft copolymerization techniques would provide a promising opportunity to widen the potential application of silk fibroin in the biomaterials field. Experimental Section Materials. Muga reeled silk, produced by following a standard degumming and reeling method14 was collected from a private farm near Jorhat, India. As the silk fibroin was processed ready for weaving, no further purification was rendered. Acrylamide (AAm) was recrystallized from methanol and then dried in vacuum over silica gel.15 Ceric ammonium sulfate (E-Merck), silica gel (CDH), H2SO4 (AR, BDH), and acetone (CDH) were used for this study. Distilled water was used to prepare all solutions. Preparation of Graft Copolymers. All the polymerization reactions were carried out in air as the use of a nitrogen atmosphere during reaction was found to have no significant contribution on conversions to graft products.16 The reaction was set up in a three necked 300 mL round-bottom flask fitted with stirrer in a temperature controlled water bath. A 1 g portion of dry silk fiber/fabric of 10 mm length was swollen with water for 15 min, and it was transferred to the reaction flask containing solutions of ceric ammonium sulfate and H2SO4 of different concentrations. The required amount of monomer (AAm) was added to the reaction system at the required temperature. The reaction time was varied from 30 to 150 min and 1 to 5 h, and the temperature of reaction was varied from 300 to 60 °C at a material to liquor ratio of 1:150. The reaction system was intermittently stirred. After the desired reaction time, silks were taken out and were then subjected to repeated extraction with boiling water and acetone to remove homopolymer and its oligomers adhering to the silks. Finally, the products were dried to constant weight and kept in desiccators over P2O5.

10.1021/ie9004755 CCC: $40.75  2009 American Chemical Society Published on Web 09/22/2009

Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009

Figure 1. FT-IR spectra of ungrafted and grafted silk fibers: (a) ungrafted, (b) 55%, (c) 66%, and (d) 73% grafted silk fiber.

The percent graft yield, total conversion, homopolymer formation, and grafting efficiency were calculated on oven dry (o.d.) weight of fiber from the increase in weight after grafting by using the following relations.17 graft yield (%) ) (B - A) × 100/A

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Characterization Methods. FT-IR spectra of ungrafted and grafted silk fibers were recorded on a Perkin-Elmer spectrometer (Model 58OB) using KBr pellets, and UV-vis spectra were recorded within the range 200-700 nm by a Shimazu UV 240 spectrometer, respectively. Thermogravimetric analysis (TGA) and differential thermogravimetric (DTG) were carried out using a Shimadzu (model 30) thermal analyzer. The masses of the samples were in the range 3.95-5.78 mg. R-Alumina was used as a reference material, and the temperature ranged from 30 to 800 °C at heating rates of 10°, 20°, and 30 °C min-1 in a static air atmosphere. DSC was obtained from a Perkin-Elmer DSC-7 with kinetic software. It consists of a compact central unit and family of cells for DSC thermogravimetry and thermochemical analysis. The heart of the system, the TA processor, functions as a combination of the control unit, computer power unit, and interface to be used through the keyboard and display. The DSC 20 standard cell is used for heat flow measurement in the temperature range around 800 °C. X-ray diffraction data were collected using a computer controlled X-ray diffractometer (type JDX-11P3A, JEOL, Japan) with a pulse-height analyzer and scintillation counter. Measuring conditions were the following: mode step; 40 kV; start angle 2°; target, Cu; 20 mA; stop angle 60°; measuring time 0.5 s; step angle 0.05. The degree of crystallinity (Kc) was found out using the following equation.18 Kc )

∫ ∫

R

0 R

0

S2Ic(S)dS S2Ic(S)dS

(1)

graft conversion (%) ) (B - A) × 100/D

where S is the magnitude of the reciprocal lattice vector and is given by

total conversion (monomer to polymer; %) ) (C - A) × 100/D

S ) 2 sin θ/λ

homopolymer formation (%) ) E × 100/D

where θ is one-half of the angle of deviation of the diffracted rays from the incident X-rays, λ is the X-ray wavelength, I(S) is the intensity of coherent X-ray scatter from a specimen (both crystalline and amorphous), and Ic(S) is the intensity of coherent X-ray scatter from the crystalline region. The water sorbency of the ungrafted and grafted silk samples was determined19

grafting efficiency (%) ) (B - A) × 100/(C - A) Where, A is the weight in grams of original silk fiber, B is the weight in grams of the grafted silk fiber after washing, C is the weight in grams of the grafted silk fiber, D is the weight in grams of the monomer, and E is the weight in grams of the homopolymer. rate of grafting(Rg) ) 1000(W/V)tm Where, W ) weight of grafted product - weight of original fiber; V ) volume of reaction mixture; t ) time of reaction in seconds; and m ) molecular mass of the monomer.

WRV(g/g) ) (wet - dry)/dry × 100

(2)

where, WRV is the water retention value in grams of water per gram of o.d. samples. The tensile properties were measured with an Elmandorf Tensile Tester by the TAPPI method, T 494 om-88.20 Drops of water were allowed to fall on the cloth samples. Water staining

Figure 2. UV-vis spectra of ungrafted and grafted silk fibers: (a) ungrafted, (b) 55%, (c) 66%, and (d) 73% grafted silk fiber.

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Figure 3. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) curves at a heating rate of 20 °C min-1: (a) ungrafted, (b) 30% grafted, (c) 40% grafted, and (d) 46% grafted. Table 1. Thermal Analysis Data: Active Decomposition Temperature and Weight Loss for Ungrafted and Grafted Silk Fibers at Heating Rates of 20 and 30 °C min-1 weight loss (%) 20 °C min-1 a

active decomposition temperature 30 °C min-1

20 °C min-1

30 °C min-1

samples

I

b

II

III

I

II

III

I

II

III

I

II

III

A

12.5 (30-140) 9.2 (30-170) 8.9 (30-175) 8.2 (30-185)

41.8 (30-170) 37.5 (180-430) 36.8 (185-440) 63.0 (195-450)

43.0 (150-390) 47.8 (430-670) 47.0 (440-675) 46.5 (450-680)

13.8 (390-640) 10.2 (30-180) 10.0 (30-185) 9.0 (30-195)

48.0 (30-160) 44.9 (185-440) 43.2 (190-445) 42.5 (450-760)

32.0 (170-400) 40.3 (400-700) 41.50 (440-740) 42.8 (445-750)

60 (endo) 85 (endo) 90 (endo) 95 (endo)






A1 A2 A3 a

A ungrafted silk fiber; A1 54%, A2 66%, and A3 73% grafted. b I, II, II pre, second, and third stages. The temperature range (°C) is in parentheses.

of the ungrafted and grafted silk fabrics was visually observed after sun drying of the wet fabrics. Results and Discussion FT-IR Properties. FT-IR spectra in Figure 1 of the ungrafted silk fiber showed characteristic bands at 1630 cm-1 (amide I), 1550 cm-1 (amide II), and 750 cm-1 (amide V) which might be attributed to the β-conformation of crystalline regions, while those bands at 1580 cm-1 (amide II) are for the random-coil conformation of fibroin molecules. The absorption band at 650 cm-1 to the R-form conformation might be from a sequential alanine polymer. Moreover, the band at 940 cm-1 might be attributed to the alanine-alanine linkages in the crystalline portion of the fiber chain. The bands at 1330 and 1360 cm-1 (amide III) and 3450 and 3270 cm-1 were caused by free (not hydrogen-bonded) -OH stretching and hydrogen-bonded -NH stretching vibrations. The grafted products of polyacrylamide

silk fiber showed slight changes in their absorption positions and also exhibited some other bands. The bands at 1640 cm-1 for the carbonyl group of amide I, 3420 cm-1 for NHstretching vibration of amide II, at 2860 cm-1 for -CH2stretching, and at 3570 cm-1 for -CONH- group were visible, thereby confirming the formation of acrylamide-grafted silk fiber.21-23 UV-vis Spectroscopic Properties. UV-vis spectroscopy is shown in Figure 2 for the ungrafted silk fiber; one shoulder form peak was observed at 213 nm for the sCONHs group of the protein due to an n f σ* transition, a hump at 330 nm was observed due to an n f π* transition of a >CdO group protein, and another low peak at 430 nm was observed due to an n f π* transition of >CHsCOsNHs of a silk protein (Figure 2). In the case of the grafted silk fiber, one hump was observed at 196 nm of σ f σ* transition due to the sCH2sC< group of acrylamide. One shoulder form peak was observed at 380 nm


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Figure 4. Differential scanning calorimetry (DSC) curves of (a) ungrafted, (b) 55%, (c) 66%, and (d) 73% grafted silk fibroin.

Figure 5. X-ray diffraction pattern of ungrafted and grafted silk fibers: (a) ungrafted, (b) 54%, (c) 66%, and (d) 73% grafted.

Table 2. (a) Thermal Analysis Data for Decomposition Temperature and Glass Transition (Tg) Values of Ungrafted and Grafted Silk Fibers at a Heating Rate of 20 °C/min: (b) X-ray Diffraction Data and Crystallinity of Ungrafted and Grafted Silk Fibers

and A3 (190 °C) depending on the increasing percent of grafting, while in the third stage, decomposition of the rest of the polymers started at 390 °C for A and increased for A1 (430 °C), A2 (440 °C), and A3 (450 °C) (Table 1), respectively. The weight loss (%) of the grafted fiber was found to be less than that of the ungrafted fiber as evident from Table 1, which showed that the initial, maximum, and final temperatures of active decomposition increased with an increase in grafting (%) of the silk fiber at all three heating rates. A similar trend was also observed at a heating rate of 30 °C min-1. It was evident from Table 1 that in both the heating rates, the initial and maximum temperatures of decomposition increased with increased grafting (%) for the silk fiber. Differential Scanning Calorometry Properties. The thermal behavior of ungrafted and grafted products of different grafting (%) were studied with the help of DSC25,27 at a heating rate of 20 °C min-1, and the thermograms are presented in Figure 4. The thermal analysis data are given in Table 2a. In case of ungrafted silk, the first broad endotherm below 100 °C is due to the evaporation of water. Two minor and broad endothermic transitions appeared at 234 and 297 °C (shoulder form), followed by a prominent endothermic peak at 370 °C. In the case of the grafted samples, the endothermic peak due to evaporation of water shifted to a higher temperature as the grafting increased. However, the AAm-grafted fibers had shown an endothermic peak at 110 °C (54% grafted), 129 °C (66% grafted), and 135 °C (73% grafted). The minor peaks (shoulder form) were shifted to 326, 340, and 352 °C for the fibers due to enthalpic change, and another endothermic peak was observed in each of the grafted fibers at 388 °C (54% grafted), 415 °C (66% grafted), and 435 °C (73% grafted). On the basis of the above DSC results, it was assumed that the endothermic peaks at 441, 465, and 477 °C for the fibers were related to the presence of the AAm polymer in the silk fiber. The glass transition temperatures (Tg) for the grafted silk fibroin were shown in Table 2a. X-ray Diffraction Studies. The X-ray diffraction patterns of silk grafted with AAm are shown in Figure 5, and the data obtained were tabulated in Table 2b. The XRD patterns exhibited the presence of both amorphous and crystalline

(a)

(b)

X-ray decomposition Tg diffraction data samples % temperature (°C) (°C) (d spacing in Å) ungrafted

grafted

54 54 54 54 66 66 66 66 73 73 73 73

92 234 297 370 110 326 388 441 129 340 415 465 135 352 435 477

175 175 175 175 250 250 250 250 267 267 267 267 273 273 273 273

2q

crystallinity (%)

6.772 (100)* 4.553 (72) 3.825 (65)

9.03 18.40 20.02

33 33 33

7.926 (100) 5.408 (85) 4.409 (79)

10.08 17.988 20.15

47 47 47

8.719 (100) 5.375 (94) 4.431 (88)

10.15 16.50 20.06

52 52 52

10.457 (100) 9.003 (92) 5.343 (81) 4.293 (65)

9.70 11.25 18.60 22.70

63 63 63 63

due to an n f π* transition of the sCONHs group of polyacrylamide and also another peak at 495 nm due to an n f π* transition of a sCHsCOsNHs group of the protein.23,24 Thermogravimetric Analysis. The thermal behavior of ungrafted and grafted products was studied from TG and DTG at heating rates of 20 and 30 °C min-1. TG and DTG curves for ungrafted and grafted products of different percentage of grafting for a heating rate of 20 °C min-1 are shown in Figure 3. The decomposition temperature ranges, the active decomposition temperatures, and percent weight losses are given in Table 1 for grafted products of different percentages of grafting.25,26 Thermal decomposition of fibers took place in three distinct stages referred to as initiation, propagation, and carbonization. All the TG curves showed an initial small mass loss step around 150 °C, which could be attributed to the removal of absorbed water. In the second stage, a major weight loss was noticed at a heating rate of 20 °C min-1 for ungrafted and grafted products (A-A3). The decomposition of the protein started at 150 °C for A, which increased for sample A1 (180 °C), A2 (185 °C),

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Scheme 1. Structural Changes in Silk Fiber during Grafting

suggested that the structural rearrangement of grafted silk took place during grafting as given in Scheme 1. In this scheme, the structure of Muga silk fibroin, which consisted mainly of the amino acid residue of glycine, alanine, serine aspartic acid, tyrosine, and arginine, has been represented in one dimension. Evaluation of Physical Properties. Tensile Properties. The values of tensile strength, elongation at break, and stretch of ungrafted and grafted silk fibers with different percentages of grafting are given in Table 3a The tensile strength, elongation at break, and stretch were found to increase with the increase of grafting (%).

Table 3. (a) Tensile Properties of Ungrafted and Grafted Silk Fibers at Different Percents of Grafting: (b) Water Retention Value (WRV) of Grafted and Ungrafted Silk Fibers (a) samples ungrafted grafted

(b)

(%)

tensile strength

elongation at break (%)

stretch (mm)

graft yield (%)

WRV (g/g)

54 66 77

3.9 6.0 6.3 6.9

16 26 31 34

30 42 44 47

54 66 73

3.80 2.44 2.03 1.87

regions.28,29 However, the crystalline regions were more prominent in the case of grafted silk fiber, in comparison with original silk fiber. From the data, it was evident that the crystallinity (%) in the grafted product was greater than that in the ungrafted one. The increase in crystalline character in the grafted products indicated that, upon grafting, the silk molecule had undergone an isomeric change, in other words, the silk molecule which was atactic had changed to isotactic after grafting with AAm using a CeIV redox initiator system. Therefore, it might be

Water Sorbency. The water sorbency of ungrafted and grafted silk fiber of different grafting (%) was determined by measuring the water retention value (WRV) by adopting the method as mentioned above. The results are recorded in Table 3b. It was found that the WRV of AAm-grafted fiber decreased with the increase of grafting (%), thereby increasing the

Table 4. Graft Copolymerization of AAm onto Antheraea assama Silk Fiber at Different Reaction Conditionsa temperature (°C) graft yield (%)

conversion of monomer to polymer (%)

rate of grafting × 105

grafting efficiency (%)

reaction time (min)

30

40

50

60

30

40

50

60

30

40

50

60

30

40

50

60

30 60 90 120 150

14 21 26 31 34

20 25 30 38 44

27 32 39 45 52

24 29 36 42 48

11.9 14.2 16.7 18.7 19.3

14.0 15.0 17.0 21.1 23.1

15.7 17.7 20.0 22.7 25.0

14.4 15.7 16.9 18.1 19.5

11.0 13.8 14.6 15.5 16.5

13.4 15.6 16.6 16.9 17.8

16.1 16.9 18.3 18.6 19.1

15.6 17.3 20.0 21.8 23.1

7.2 5.4 4.5 4.0 3.5

10.4 6.5 5.2 4.9 4.5

14.0 8.3 6.7 5.8 5.4

12.5 7.5 6.2 5.4 5.0

a The results are averages of three readings. Silk fiber 1 g, AAm 1 M, cerric ammonium sulfate 12 × 10-3 M, and H2SO4 acid 12 × 10-2 M, material to liquor ratio 1:150.

Table 5. Graft Copolymerization of AAm onto Antheraea assama Silk Fabric at Different Reaction Conditionsa temperature (°C) graft yield (%) reaction time (h) 30 40 50 60 1 2 3 4 5

17 22 29 35 32

24 31 38 47 40

30 37 44 50 46

28 35 42 48 45

conversion of monomer to polymer (%)


graft conversion (%)


30

40

50

60

30

40

50

60

30

40

50

60

30

40

50

60

15.3 17.1 19.8 20.7 20.0

16.7 19.6 20.8 21.9 21.1

18.5 20.3 21.7 23.0 22.3

17.9 19.5 20.2 21.9 21.1

07.5 09.2 11.3 11.9 12.6

11.5 13.0 14.2 15.7 14.2

12.4 13.8 15.2 16.8 15.1

11.9 13.3 14.0 16.0 15.3

1.8 2.0 2.8 3.2 3.0

2.5 2.9 3.8 4.5 4.0

3.6 4.2 4.8 5.7 5.1

2.7 3.4 4.0 4.8 4.2

5.0 4.2 3.1 2.8 2.4

6.5 5.3 4.5 3.8 3.3

8.9 6.7 5.6 4.9 4.1

7.8 5.5 4.9 4.0 3.6

The results are averages of three readings. Silk fabric 1 g, AAm 1 M, cerric ammonium sulfate 12 × 10-3 M, and H2SO4 acid 12 × 10-2 M, material to liquor ratio 1:150. a


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a

Table 6. Graft Copolymerization of AAm onto Antheraea assama Silk Fiber at Different Reaction Conditions time (h) graft yield (%)




monomer concn (M)

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

0.5 1.0 1.5 2.0 2.5

21 28 33 48 41

27 35 49 60 54

32 59 71 85 70

39 67 89 97 78

34 64 81 89 73

3.9 2.6 2.1 1.9 1.5

5.1 3.5 3.1 2.8 2.0

6.0 5.5 4.5 4.0 2.4

7.3 6.3 5.6 4.5 2.7

6.4 6.0 5.1 4.2 2.6

16.0 19.4 23.5 24.6 24.0

19.8 21.3 26.5 28.7 27.3

21.9 24.0 27.9 32.6 28.2

23.5 24.5 31.9 35.5 32.7

22.6 24.0 28.8 32.8 29.3

5.4 7.2 8.5 12.5 10.6

3.5 5.2 6.3 7.8 7.0

2.7 5.1 6.1 7.3 5.9

2.5 4.3 5.7 6.3 5.0

1.7 3.3 4.2 4.6 3.8

a The results are averages of three readings. Silk fiber 1 g, ceric ammonium sulfate 15 × 10-3 M, and H2SO4 acid 15 × 10-2 M, temperature 55 °C, material to liquor ratio 1:150.

Table 7. Graft Copolymerization of AAm onto Antheraea assama Silk Fiber at Different Reaction Conditionsa time (h) graft yield (%)

homopolymer formation (%)



CeIV concn (M)

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

15 20 25 30 35

28 33 36 39 34

35 38 42 48 40

59 62 71 78 68

67 70 75 82 71

64 66 73 79 69

11.9 13.9 16.2 19.2 15.7

13.4 16.7 20.9 23.6 18.4

17.5 19.7 35.3 37.5 35.9

19.4 21.1 36.9 39.0 37.2

18.7 18.7 35.8 38.2 36.4

19.4 19.4 23.5 24.8 23.2

21.3 21.3 24.1 25.5 23.9

24.0 24.0 25.6 26.2 27.9

24.5 24.5 26.2 26.6 28.4

24.2 24.2 25.9 26.4 28.1

7.2 7.2 9.3 10.1 8.8

5.2 5.2 6.2 6.9 6.0

5.1 5.1 6.1 6.7 5.9

4.3 4.3 4.8 5.3 4.6

3.3 3.3 3.8 4.1 3.5

a The results are averages of three readings. Silk fiber 1 g, AAm 1 M, and H2SO4 acid 15 × 10-2 M, temperature 55 °C, material to liquor ratio 1:150.

Table 8. Graft Copolymerization of AAm onto Antheraea assama Silk Fiber at Different Reaction Conditionsa time (h) graft yield (%)

total conversion (%)



H2SO4 concn (M)

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

15 17 19 21 23

28 30 34 38 32

35 37 41 44 39

59 61 66 72 63

67 69 69 79 70

64 63 63 74 67

13.5 13.9 13.9 16.3 14.5

15.4 16.4 16.4 17.4 16.0

23.1 23.2 23.2 26.1 23.5

25.7 26.6 26.6 27.6 25.1

24.7 24.8 24.8 26.7 24.4

2.6 2.8 2.8 3.6 3.0

3.3 3.6 3.6 4.1 3.7

5.5 5.7 5.7 6.7 5.9

6.3 6.5 6.5 7.4 6.7

6.0 6.1 6.1 6.9 6.3

7.2 7.8 7.8 9.9 8.3

5.2 5.4 5.4 6.6 5.7

5.1 5.3 5.3 6.2 5.4

4.3 4.4 4.4 5.1 4.5

3.3 3.4 3.4 3.8 3.5

a

The results are averages of three readings. Silk fiber 1 g, AAm 1 M, and CeIV 15 × 10-3 M, temperature 55 °C, material to liquor ratio 1:150.

Table 9. Kinetic Parameters of Ungrafted and Grafted Silk Fibers at a Heating Rate of 20 °C min-1 20 °C min-1 I

II

samplesa

E (kJ mol-1)

A (s-1)

SED (s-1)

E (kJ mol-1)

A (s-1)

SED (s-1)

A A1 A2 A3

25.098 32.324 35.546 41.720

0.113 0.312 0.720 0.823

0.018 0.001 0.002 0.004

27.109 36.339 39.454 47.192

0.145 0.390 1.398 1.647

0.002 0.001 0.003 0.002

a

A ungrafted silk fiber; A1 54%, A2 66%, and A3 73% grafted.

hydrophobic nature of the fiber. The decrease in WRV might be due to the decreased cohesive force of the highly swollen fibers. Water Staining. Drops of water were allowed to fall on the cloth samples. Water staining of the ungrafted and grafted silk fabric was visually observed after sun drying of the wet fabrics. It was found that the grafted silk fabrics did not retain any stain. Effect of Temperature and Time. The effect of reaction temperature on grafting of AAm onto silk fibers/fabrics was carried out at four different temperatures ranging from 30 to 60 °C. The weight gain increased steadily up to 50 °C, and beyond that, the rate gradually slowed down (Tables 4 and 5). The increased in grafting (%) could be ascribed to greater activation energy. As the temperature of the reaction increased, the swellability of the fiber/fabric is greatly enhanced, and as such, diffusion of the monomer from the solution phase to the fiber/fabric phase took place.30 The effect of reaction time on

graft yield (%) was studied for reactions carried out for 30-150 min and 1-5 h durations in the case of silk fiber and fabric, respectively. The graft yield (%) increased with the increase in the time of reaction up to 150 min, and 4 h after that, the rate of grafting slowed down (Tables 4 and 5). It may be concluded that this was due to depletion in the available active centers on the substrate backbone as the reaction proceeded.31 Effect of Monomer, Initiator, and Acid Concentrations. The graft copolymerization of AAm onto silk fiber was carried out by varrying the monomer concentrations from 0.5 to 2.5 mol/L (M) in the case of the CeIV initiator system and keeping all other conditions of reactions constant. With the increase in monomer concentrations up to 2 M, the rate of grafting which increased gradually and decreased is shown in Table 6. This might be due to the fact that, at a certain monomer concentration, combination of monomer probably took place with silk molecules. When the concentration of the polyacrylamide radical (PAAm•) increased, the rate of their combination also increased faster than the rate of their combination with silk molecules.30 With concentrations of initiator, CeIV, varying from 15 to 35 × 10-3 M, it was observed that the rate of grafting (%) increased progressively with the increasing initiator concentration until 30 × 10-3 M, and decreasing data are shown in Table 7. This would be expected since, at higher concentrations, [Ce4+] is known to affect grafting by termination of growing grafted chains.19 The concentrations of H2SO4 were also varied from 15 to 23 × 10-3 M. The extent of grafting (%) increased progressively up to 21 × 10-3 M and thereafter tended to decrease (Table 8).

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Scheme 2. Schematic Representation of Anthearea assama Silk Fibroin Molecule in One Dimension: Reaction Mechanism Representation

This was explained by the fact that the ceric ion in water was believed to react in the following manner.31 Ce4+ + H2O h [Ce(OH)3]3+ + H+ 2[Ce(OH)3]3+ h [Ce-O-Ce]6+ + H2O

(3)

Thus, the ceric ion exists as Ce4+, [Ce(OH)3]3+, and [Ce-O-Ce]6+ in water solution. With an increase in sulphuric acid concentrations beyond 21 × 10-3 M, the equilibria in above equations shift toward formation of more and more Ce4+ and [Ce(OH)3]3+. Therefore, no further reactions take place, hence the decrease in grafting (%). Kinetic Studies. The retrieval of kinetic parameters from weight loss versus temperature data could be carried out by using various methods.34,35 In the present work, the well-known Coats and Redfern method36 was used for retrieving kinetic parameters from dynamic thermogravimetry. The general correlation equation used in Coats and Redfern method is Log10[1 - (1 - R)]1-n/Τ2(1 - n) ) log10[AR/REa(1 - 2RT/Ea)] - E/2.3RT

(4)

where, R is the reaction decomposed at temperature T, n is the order of the reaction, A is the frequency factor (s-1), a is the heating rate (K min-1), R is the gas constant (kJ mol-1 K-1), T

is the temperature (K), and E is the activation energy (kJ mol-1). A computer program in Fortran 77 was used for the linear leastsquares analysis with the Gauss-Jordan subroutine and applied to evaluate n, ∆E, A and SED, simultaneously. The procedure basically involved stepwise change of the order of the reaction, n (over a range of 0.6-1.6), to determine the SED in leastsquares estimates of the parameters, ∆E and A. The data were found to fit well for a fast order reaction and are given in Table 9. The values of activation energy and frequency factors for grafted products were higher than those of the ungrafted one. These values increased with increase in the molecular weights, i.e., increase in the grafting (%). Structure Determination of Antheraea assama Silk Fiber. The chemical composition of Muga silk fibroin consisted predominantly of amino acid residues of glysine, alanine, and serine (80.5 mol %) with small side chains.37 These three amino acids form the fibrous component of most of the known silks. The other amino acids present in the Muga silk fibroin are aspartic acid, threonine, glutamic, valine, cystine, methionine, leucine, tyrosine, phenylalanine, lysine, proline, tryptophane, histidine, and arginine.1 The polypeptide chains in the silk are arranged in β-antiparallel38 conformation, and due to the presence of functional groups like -OH, -NH2, -COOH, etc. in the constituent amino acids, the initiating radical may attack the polypeptide backbone as well as the side chain functional


groups, thereby facilitating formation of macroradicals during grafting. In the system, the mechanism of formation of silk macroradicals (Ce4+) by interaction with H2SO4 and the formation of silk macroradicals by graft copolymerization may be explained as shown in Scheme 2. Conclusion Graft copolymerization of acryl amide onto nonmulberry silk fiber (Antheraea assama) by the CeIV redox system could be carried out in the presence of air, and optimum results were obtained when the reaction time was 4 h at 55 °C. The grafted silk fibers were found to be thermally more stable than those of the ungrafted one. It was also observed that with the increase in grafting, the water retention value decreased. The tensile strength and elongation of the grafted fibers were found to increase with increasing grafting. The water staining character of the silk was also minimized. Acknowledgment The authors thank Dr. Raju Khan, Analytical Chemistry Division, North East Institute of Science & Technology (NEIST), Jorhat, Assam, for scientific discussions. Literature Cited (1) Hazarika, L. K.; Saikia, C. N.; Kataky, A.; Bordoloi, S.; Hazarika, J. Evaluation of physico-chemical characteristics of silk fibres of Antheraea assama reared on different host plants. Bioresour. Technol. 1998, 64, 67. (2) Jin, H. J.; Park, J.; Valluzzi, R.; Cebe, P.; Kaplan, D. L. Structure and properties of silk hydrogels. Biomacromolecules 2004, 5, 711. (3) Asakura, T.; Sugino, R.; Yao, J. M.; Takashima, H.; Kishore, R. Comparative Structure Analysis of Tyrosine and Valine Residues in unprocessed Silk Fibroin (Silk I) and in the processed Silk Fibre (Silk II) from Bombyx mori using Solid State 13C, 15N and 2H NMR. Biochemistry 2002, 41, 4415. (4) Altman, G. H.; Diaz, F.; Jakuba, C.; Calabro, T.; Horan, R. L.; Chen, J. S.; Lu, H.; Richmond, J.; Kaplan, D. L. Silk-based biomaterials. Biomaterials 2003, 24, 401. (5) Tsukada, M.; Freddi, G.; Crighton, J. S. Structure and compatibility of poly (vinyl alcohol)-silk fibroin (PVA/SF) blend films. J. Polym. Sci. B: Polym. Phys. 1994, 32, 243. (6) Liang, C. X.; Hirabayashi, K. Improvements of the Physical Properties of Fibroin Membranes with Sodium Alginate. J. Appl. Polym. Sci. 1992, 45, 1937. (7) Yang, G.; Zhang, L. N.; Liu, Y. G. Structure and microporous formation of cellulose/silk fibroin blend membranes: I. Effect of coagulants. J. Membr. Sci. 2000, 177, 153. (8) Chen, X.; Li, W. J.; Zhong, W.; Lu, Y. H.; Yu, T. Improvements of the physical properties of fibroin membranes with sodium alginate. J. Appl. Polym. Sci. 1997, 65, 2257. (9) Tsukada, M.; Freedi, G.; Shiozaki, H.; Pusch, N. Changes in physical properties of methacrylonitrile (MAN) grafted silk fibers. J. Appl. Polym. Sci. 1993, 49, 593. (10) Tsukada, M.; Shiozaki, H.; Gotoh, Y.; Freedi, G. Physical properties of silk fibers treated with ethylene glycol diglycidyl ether by the Pad/batch method. J. Appl. Polym. Sci. 1993, 50, 1841. (11) Tsukada, M.; Shiozaki, H.; Gotoh, Y.; Freedi, G.; Crighton, J. S. Physical characteristics of silk fibers modified with dibasic anhydrides. J. Appl. Polym. Sci. 1994, 51, 345. (12) Das, A. M.; Saikia, C. N. Graft copolymerization of methylmethacrylate onto non-mulberry silk-Antheraea assama using potassium permanganate-oxalic acid redox system. Bioresour. Technol. 2000, 74, 213. (13) Das, A. M.; Saikia, C. N.; Hussain, S. Grafting of methyl methacrylate (MMA) onto Antheraea assama silk fiber. J. Appl. Polym. Sci. 2001, 81, 2633. (14) Choudhury, S. N. Silk and Sericulture, first ed.; Directorate of Sericulture: Assam, India, 1992.

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(15) Samal, R. K.; Nanda, C. N.; Satrusallya, S. C.; Nayak, B. L.; Suryanarayana, G. V. Grafting of vinyl monomers onto silk fibers: Trivalentmanganese-initiated graft copolymerization of acrylamide onto silk fibers. J. Appl. Polym. Sci. 1983, 28, 1311. (16) Das, A. M.; Saikia, C. N. Property modification of Antherea assama silk fibre through graft copolymerization. Ind. J. Fibre Tex. Res. 2002, 27, 194. (17) Rabek, J. F. Experimental methods in polymer chemistry; John Wiley & Sons: London, 1980. (18) Fernandez, M. J.; Casinos, I.; Guzman, G. M. Effect of the way of addition of the reactants in the graft copolymerization of a vinyl acetatemethyl acrylate mixture onto cellulose. J. Appl. Polym. Sci. A: Polym. Chem. 1990, 28, 2275. (19) Singha, A. S.; Shama, A.; Thakur, V. K. Pressure induced graftco-polymerization of acrylonitrile onto Saccharum cilliare fibre and evaluation of some properties of grafted fibre. Bull. Mater. Sci. 2008, 31, 7. (20) TAPPI. Technical Association of Pulp and Paper Industry; Tappi Press: Atlanta, GA, 1993. (21) Silverstein, R. M.; Bossless, G.; Clayton, M.; Terence, C. Spectrometric Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991. (22) Barnwell, C. N. Fundamentals of Molecular Spectroscopy, 3rd ed.; Tata McGraw-Hill: New Delhi, 1972. (23) Scheinmann, F. An Introduction to Spectroscopic Methods for the Identification of Organic Compounds; Pergamon Press: New York, 1970. (24) Hirayana, K. Hand Book of UltraViolet and Visible Absorption Spectra of Organic Compounds; Plenum Press Data Div.: New York, 1967. (25) Isil, A.; Gulin, S. P.; Saadet, O. Thermal oxidative degradation kinetics and thermal properties of poly(ethylene terephthalate) modified with poly(lactic acid). J. Appl. Polym. Sci. 2008, 109, 2747. (26) Wang, J.; Wenhui, Wu.; Zhihui, L. Kinetics and thermodynamics of the water sorption of 2-hydroxyethyl methacrylate/styrene copolymer hydogels. J. Appl. Polym. Sci. 2008, 109 (5), 3018. (27) Kessler, M. R.; White, S. R. Cure kinetics of the ring-opening metathesis polymerization of dicyclopentadiene. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2373. (28) Spinger, V. Worked Examples in X-ray Analysis (Supplied with the Computer Controlled XRD; Type JDX-11P 3A, JEOL, Japan, Spinger: Berlin, 2006. (29) Singha, A. S.; Kaith, B. S.; Chauhan, A.; Misra, B. N. Mechanical properties of natural fibre reinforced polymer composites. J. Polym. Mater. 2006, 23, 3456. (30) Liu, Z.-T.; Chang’an, S.; Liu, Z.-W.; Jian, L. Adjustable wettability of methyl methacrylate modified ramie fibre. J. Appl. Polym. Sci. 2008, 109 (5), 2888. (31) Jian, M. J.; Hui, J. Z.; Guang, L.; Jin, J. H.; Sheng, L. Y. Poly (P-phenylene benzoxazole) fibre chemically modified by the incorporation of sulfonate groups. J. Appl. Polym. Sci. 2008, 109, 3133. (32) Hongehun, Li.; Jincai, Li.; Yuetao, Z.; Ying, Mu. Ethylene/a-olefin copolymerization using diphenylcyclopentadienyl-phenoxytitanium dichloride/ Al(iBu)3/[Ph3C][B(C6F5)4] catalyst systems. J. App. Polym. Sci. 2008, 109, 3030. (33) Tripathy, S. S.; Jena, S.; Singh, B. C. Graft copolymerization of methylmethacrylate onto silk fibers with Ce (VI)-sucrose redox system. J. Appl. Polym. Sci. 1983, 28, 1811. (34) Ali, H.; Ebrahim, V.-F. Kinetic study of methacrylate copolymerization systems by thermoanalysis methods. J. Appl. Sci. 2008, 109, 3302. (35) Budrugeac, P.; Segal, E. On the nonlinear isoconversional procedures to evaluate the activation energy of nonisothermal reactions in solids. J. Chem. Kinet. 2003, 36, 87. (36) Coats, A. W.; Redfern, J. R. Kinetic Parameters from Thermogravimetric Data. Nature 1964, 68, 201. (37) Freddi, G.; Gotoh, Y.; Mori, T.; Tsutsui, I.; Tsukada, M. ChemicalStructure and Physical-Properties of Antheraea-Assama Silk. J. Appl. Polym. Sci. 1994, 52, 775. (38) Finer, I. Organic Chemistry: Stereochemistry and the Chemistry of Natural Products; 5th Ed., Longman Group Ltd.: U.K., 1975.

ReceiVed for reView March 23, 2009 ReVised manuscript receiVed August 24, 2009 Accepted September 13, 2009 IE9004755

Some Physical Properties and Structure Determination of Vinyl

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