Multicomponent Polymer Systems

21 Properties of Graft and Block Copolymers of Fibrous Cellulose

Downloaded by CORNELL UNIV on June 1, 2017 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0099.ch021

JETT C. ARTHUR, JR. Southern Regional Research Laboratory, Southern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture, New Orleans, La. 70119

Graft and block copolymers of cotton cellulose, in fiber, yarn, and fabric forms, were prepared by free-radical initiated copolymerization reactions of vinyl monomers with cellulose. The properties of the fibrous cellulose-polyvinyl copolymers were evaluated by solubility, ESR, and infrared spectroscopy, light, electron, and scanning electron microscopy, fractional separation, thermal analysis, and physical properties, including textile properties. Generally, the textile properties of the fibrous copolymers were improved as compared with the properties of cotton products.

"Cibrous celluloses, both as natural fibers and regenerated cellulosic fibers, comprise more than two-thirds of the world's textile fibers which include apparel, household, and industrial products. Cotton cellulose, even i n developed countries, is still the major textile fiber. In the United States cotton's percentage share of the textile fiber market has declined i n recent years; however, about four billion pounds of cotton are used annually i n America i n textile products. The blending of cellulosic fibers with man-made fibrous polymers to obtain textile fabrics with desired properties is commonly practiced. Other approaches have been to alter the properties of cellulosic fibers through chemical modification, such as acetylation, physical modification, such as mercerization, and copolymerization with vinyl monomers (3, 4, 9, 11,17). The modification of the properties of fibrous cotton cellulose through free-radical initiated copolymerization reactions with vinyl monomers has been investigated at the Southern Laboratory for a number of years. Both graft and block copolymers are formed. Under some experimental conditions the molecular weight of the polyvinyl polymer, covalently 321 Platzer; Multicomponent Polymer Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

322

M U L T I C O M P O N E N T P O L Y M E R SYSTEMS

Table I.

Solubility of Cellulose in Polyacrylonitrile Copolymers of Cellulose in Cupriethylenediamine (0.5M) at 25°C

Copolymer"


A B C D E

Fraction of Cellulose Soluble,

N i i r 0 9 m

%

Total

99 48 97 0 0

6.6 4.8 13.0

—

—

'

%

Insoluble Fraction

—

10.2

—

11.5

—

° A, purified cotton cellulose; B, cellulose (75%)-polyacrylonitrile (25%) copolymer (y-radiation initiated); C, cyanoethylated cellulose (D.S. 0.7); D, cyanoethylated cellulose (D.S. 0.7) (62%)-polyacrylonitrile (38%) copolymer (y-radiation initiated); E, cellulose (75%)-polyacrylonitrile (25%) copolymer (eerie ion initiated).

linked to cellulose, may be equal to or greater than the molecular weight of the cellulose molecule (12, 42). Previously, the basic mechanisms and principles involved i n the free-radical reactions of cellulose with vinyl monomers were discussed (1, 2, 8,10,19, 20, 25,40). In this chapter we summarize the properties of graft and block copolymers of fibrous cotton cellulose, including chemical structure, morphology, and physical properties. Some comparisons of the effects of free-radical initiation of copolymerization reactions of cellulose with vinyl monomers on the properties of cotton products are made. Chemical Structure Solubility. The effects of free-radical initiation of acrylonitrile copolymerization with cellulose on the solubility of the cellulose in the copolymer product i n cupriethylenediamine are shown in Table I. The solubility of the cellulose in the product prepared by the radiation method was greater than the solubility of the cellulose i n the product prepared by the eerie ion method (34). W e have reported previously that the moles of cellulose per mole of grafted polyacrylonitrile i n products prepared by the radiation method, using aqueous Z n C l , ranged from about 5 to 86 and in products prepared by the eerie ion method was about 0.4 (42). The higher molecular degree of substitution of polyacrylonitrile in the latter product probably accounts for the lower solubility of the cellulose i n the product. Cyanoethylation of cellulose plus copolymerization of acrylonitrile with the modified cellulose, initiated by radiation, also gave a product i n which the cellulose was not soluble i n cupriethylenediamine. In this case probably most of the hydroxyl groups on the cellulose molecule, normally accessible for complexing with cupriethylenediamine and i n effecting dissolution of the cellulose, reacted (15, 16, 34). 2

Platzer; Multicomponent Polymer Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1971.


21.

ARTHUR, J R .

323

Copolymers of Fibrous Cellulose

Solubility of cellulose in the products has also been used to indicate the presence of covalent bonds between cellulose and the polyvinyl copolymer. T h i n sections of fibrous cellulosic copolymers, as prepared for electron photomicrography, are examined both before and after successive extractions with solvents for cellulose and for the polymer. The presence of undissolved cellulose and polymer in the extracted thin section is interpreted as evidence for the presence of covalent bonds and for grafting (52). Instrumental Methods. Infrared spectroscopy has often been used to investigate the chemical structure of cellulose copolymers. Since freeradical initiated reactions of cellulose usually involve oxidative depolymerization of the cellulose molecule, increases in infrared absorption arising from the increase in concentration of C==0 groups are recorded (5). Attempts to identify covalent links between cellulose and polyvinyl polymer by recording infrared spectral data have been qualitatively successful. Typical infrared data for cellulose—polyacrylonitrile copolymers are shown in Table II. Cellulose, crystalline lattice type I, was copolymerized with acrylonitrile. The products were extracted with 2V,N-dimethylformamide to remove homopolymer prior to their examination. The increase in the concentration of the characteristic C = N group of polyacrylonitrile is readily detected. For both copolymers B and E decreases in O — H and C — H concentrations are recorded, which may indicate covalent bonds between cellulose and polyacrylonitrile (14). Previously, we reported that the mechanisms of free-radical initiation, as shown by E S R spectroscopy data, were probably (1) dehydrogenation, and to a lesser extent depolymerization, in the case of y-irradiation and (2) complexing of eerie ion with O H groups on C and C of the cellulose molecule in the case of redox initiation (J, 5, 6). Polarized infrared spectral data of cellulose copolymer products gave similar results (56). 2

Table II.

3

Infrared Spectral Data of Polyacrylonitrile Copolymers of Cellulose Optical Density/mg. Cellulose

Copolymer"

2.8-3.4* 0-H stretching

34* C-H stretching

445* C-N stretching

7.25* C-H deformation

8.6* C-OH deformation

A B C D E

0.534 0.363 0.371 0.475 0.373

0.213 0.169 0.183 0.234 0.221

0.000 0.091 0.063 0.157 0.061

0.281 0.243 0.246 0.325 0.224

0.457 0.357 0.384 0.475 0.331

a

Footnote same as for Table I.


324


Table III.

Copolymer"

Density of Polyacrylonitrile Copolymers of Cellulose

1

Polyacrylonitrile, %

A B C D

0 14 19 25

Breaking Strength of Fibrous Copolymer, X 10~ gram

Density, grams/ml

4.5 4.1 4.0 4.4

1.533 1.462 1.434 1.421

z

°y-Radiation initiated copolymerization. Monomer solution: 32 parts of acrylonitrile in 68 parts of aqueous 80% ZnCl*. Dosage: A, O; B, 2.1 X 10 ev/gram; C, 3.1 X 10 ev/gram; D, 4.2 X 10 ev/gram. 19

w


19

* Applications of E S R spectroscopy to investigations of free-radical initiated copolymerization reactions of cellulose with vinyl monomers have been reported ( I , 2 ) . N M R and infrared spectroscopy have been used to examine products obtained from degradation of cellulose copolymers to characterize the nature of the chemical bonds between cellulose and polyvinyl polymer (35, 42). Covalent bonds were indicated i n most cases. X-ray examination of cellulose, which had been y-irradiated, showed no change i n the degree of crystallinity of the cellulose (2,5). Similarly, x-ray examinations of cellulose copolymers have not demonstrated conclusively that the formation of copolymers decreased the crystallinity of cellulose (54,55,57). Fractional Separation. The densities of cellulose copolymers have been used to differentiate between copolymers and mixtures of cellulose and homopolymers. Typical variations i n the densities of fibrous cellulose—polyacrylonitrile copolymers are shown i n Table III. The whole graft copolymer, which contained the highest percentage of polyacrylonitrile, had the lowest density. The values are i n fairly good agreement with those that would have been predicted from the known densities of cellulose and amorphous polyacrylonitrile. Increased concentration of amorphous polyacrylonitrile i n and decreased density of the copolymer d i d not significantly affect the breaking strength of the fibrous copolymer (13). The degradation of the cellulose fraction of the copolymer and subsequent recovery of the polyvinyl polymer have often been used to characterize the polymer. F o r example, cellulose may be acetylated and acid hydrolyzed to remove it from the copolymer. Then the recovered polymer can be dissolved, i n solvent normally used for the polymer, and. the molecular weight of the polymer determined viscometrically (12, 42). As reported previously for polymers, such as polyacrylonitrile, a functional group on the polymer may be altered during the fractionating. These changes have been detenmned by infrared spectroscopy. F o r free-



21.

325


ARTHUR, JR.

radical initiated copolymerization reactions, the molecular weights of polyacrylonitrile copolymers may range from about 30,000 to more than 1,000,000 (42). F o r polystyrene copolymers molecular weights of about 300,000-500,000 have been reported (12). The initial molecular weight of cellulose during free-radical initiated copolymerization reaction may be as high as 700,000 (4, 5). D u r i n g the reaction, some oxidative depolymerization of cellulose occurs, so that i n many instances the copolymer has a higher molecular weight than the cellulose to which it is chemically bonded (J, 5 ) . W h e n copolymers are prepared by the free-radical initiation of reactions of irradiated cellulose (on which the free-radical site is located) with binary mixtures of vinyl monomers, fractionation of cellulose and IOr

^^1

i O"

W

i 60*

i

i 90*

i 120*

TEMPERATURE,

150* #

|

1

BO*

210*

C

Figure 1. Effect of temperature on the stiffness of fibrous ceUutose-^polyvinyl copolymers A; purified cotton B:fibrouscelltdose-polyacrylonitrile C: cyanoethylated cotton D:fibrouscyanoethylated cellulose-polyacrylonitrile E:fibrouscellulose-polystyrene


326



1600-

80»

I60»

0»

UPPER CURVE OF EACH GRAPH-ENERGY EXPENDED. LOWER CURVE OF EACH GRAPH-ENERGY RECOVERED

80*

160*

0*

80*

ISO*

TEMPERATURE °C

Figure 2. Effect of temperature on the relationship between energy expended to elongate fibrous cellulose-polyvinyl copolymers and energy recovered on relaxation of the copolymers. (See Figure 1 for identification of copolymers.) polymer indicates that grafted block polymers may be bonded to cellulose. The composition of the grafted polymer depended on the composition of the binary mixture of monomers but was not necessarily the same as that composition. For a given binary mixture the composition of the grafted polymer was independent of the extent of graft copolymerization (37).

Thermal Analysis. Fibrous cellulose copolymers exhibit softening or second-order transition temperatures, as shown i n Figure 1. I n both purified cotton (Figure 1A) and cyanoethylated cotton (Figure 1 C ) , the stiffnesses of the fibers decreased slightly with increasing temperature. The presence of a small amount of moisture in these fibers probably had a plasticizing effect which resulted i n these relationships. The stiffnesses of fibrous cellulose-polyacrylonitrile (Figure I B ) , cellulose-polystyrene (Figure I E ) , and cyanoethylated cellulose-polyacrylonitrile (Figure I D ) decreased with increasing temperature. These changes indicate that the cellulose copolymers became soft and extensible with increasing tern-



21.

ARTHUR, JR.


327

perature and passed through second-order transition temperatures. C y anoethylated cellulose-polyacrylonitrile exhibited the lowest secondorder transition temperature of the copolymers examined probably because of the high amorphous content of the copolymer. Fibrous cellulose—polystyrene copolymers showed a drop i n resiliency index and stiffness with increasing temperature at about 100°C; cellulose-polyacrylonitrile and cyanoethylated cellulose-polyacrylonitrile, at about 80°-100°C (14,33). O n relaxation the recovery of energy expended to elongate the fibrous copolymers to about 1.5% of their initial length at temperatures ranging from 21° to 200°C is shown in Figure 2. The lined sections in each graph of Figure 2 show the energy expended during elongation at a given temperature that was not recovered during relaxation at that temperature. Summation of the energy relationships over the entire temperature range show that the recovery of energy expended ranged from 38 to 4 3 % for cellulose, cyanoethylated cellulose, cellulose-polyacrylonitrile, and cellulose-polystyrene. F o r cyanoethylated cellulose-polyacrylonitrile copolymer the recovery of energy was about 66%. These results indicate that it is possible to prepare cellulose-polyvinyl copolymers which have thermoplastic and/or thermoelastic properties (24, 33). Elastomers, prepared by free-radical initiated copolymerization of ethyl acrylate with cellulose to several hundred percent extent of grafting of poly (ethyl acrylate) onto cellulose, exhibited rubber-like behavior and second-order transition temperatures. Cellulose—poly (ethyl acrylate) elastomers had transition temperatures below —35°C, about —20°C, and below 5 ° C when measured i n ethyl acetate, dry air, and water, respectively (43, 44). Morphology As reported previously, the morphology of fibrous cellulose-polyvinyl copolymers, determined b y electron microscopy, depends on the method of free-radical initiation of the copolymerization reaction, the experimental conditions during the reaction, and the type of vinyl monomer used. Variations i n the shape of the fibrous copolymer cross section, in layering effects i n the copolymer structure, and in location and distribution of the polyvinyl polymer within the fibrous structure were shown ( J , 2, 7,29,52). Recently, scanning electron microscopy has been used i n our laboratory to investigate the effects of abrasion on the morphology of fibrous cellulose copolymers (36). F o r example, cotton cellulose was woven into fabric form (print cloth construction and weight). Cellulose copolymer fabrics were prepared by irradiating a sample of this fabric, followed b y copolymerization of the irradiated fabric with a binary mixture of acrylo-


M U L T I C O M P O N E N T

P O L Y M E R

SYSTEMS


328

Figure 3. Scanning electron microphotograph of cellulose fabric sample before (A) and after (B) flex abrasion nitrile and butyl methacrylate (37). Under a flex abrasion test, untreated fabric ruptured after 2000 cycles. Scanning electron microphotographs of the surfaces of samples of this fabric and the abraded fabric are shown


ARTHUR, J R .


329


21.

Figure 4. Scanning electron microphotograph of ceuulose-^polyacrylonitrile-poly(butyl methacrylate) copolymer fabric sample before (A) and after (B) flex abrasion in Figure 3. As shown in Figure 3B, the fibers were ruptured and splintered by abrasion. Under the same flex abrasion test, the cellulose copolymer fabric was not ruptured even after 24,000 cycles. The surfaces


330

M U L T I C O M P O N E N T

P O L Y M E R

S Y S T E M S

of the copolymer fabric and the abraded copolymer fabric are shown in scanning electron microphotographs in Figure 4. The copolymer fibers (Figure 4 A ) are smoother than the untreated fibers (Figure 3 A ) . T h e abraded copolymer fibers have been sheared but appear to be fused together (Figure 4 B ) . They are not ruptured as in the case of the untreated fibers (Figure 3 B ) . This probably accounts for the high flex abrasion resistance of the copolymer fabric as compared with that of the untreated fabric.


Physical Properties Free-Radical Initiation. The effects of the method of free-radical initiation of the copolymerization reaction of acrylonitrile with fibrous cellulose on the properties of copolymer fabrics are shown in Table IV (31). A t the same level of polymer add-on (25-27%), the molecular weight of Table IV. Effect of Method of Initiation of Copolymerization Reaction on the Properties of Polyacrylonitrile—Cotton Copolymer Fabrics (Print Cloth) Copolymerization Reaction Conditions'

1

Property

Control

Grafted polymer Add-on, % — Molecular weight of grafted polymer, X 10~ — Breaking strength, X 10~ gram 1.9 Elongation at break, % 22 Tear resistance, gram 573 Flex abrasion, sample/control 1.0 Flat abrasion, sample/control 1.0 Shape of fibrous cross-section kidney Polymer distribution in fibrous cross-section — 5

4

A

B

C

D

E

25

27

25

26

26

5.9

5.1

0.33

0.84

0.84

2.3

2.3

1.8

1.9

1.8

26

34

21

18

22

507

650

447

393

413

1.7

2.4

1.1

1.4

1.0

6.1

6.3

2.0

3.4

2.3

kidney

round

kidney

kidney

kidney

outer layers

uniform

uniform

uniform

uniform

" A : post-irradiation grafting; y-radiation 1 megarad; cellulose immersed in 80% ZnCl . B: simultaneous irradiation grafting; -r-radiation 0.2 megarad; cellulose immersed in 32% acrylonitrile in 80% aqueous ZnCl*. C : simultaneous irradiation grafting; Y-radiation 1 megarad; cellulose immersed in 30% acrylonitrile in iV,iV-dimethylformamide. D : eerie ion (0.005M) in H N 0 (0.05M) grafting; 4% acrylonitrile. E : ferrous ion (0.1%) hydrogen peroxide (0.03%) grafting; 8% acrylonitrile. 2

3



21.

ARTHUR, J R .


331

the grafted polymer varied from 33,000 to 590,000 and depended on the method of free-radical initiation and experimental conditions. There is no obvious relationship between molecular weight of the grafted polymer and the properties of the copolymer fabrics. Copolymerization reaction condition I V B gave copolymer fabrics with increased breaking strength, tear resistance, and flex and flat abrasion resistances as compared with the control fabric and the other copolymer fabrics. This improvement may be caused partly by effects of the experimental conditions on the morphology of the copolymer fabrics. As reported, the fibrous cross section was rounded, and the grafted polymer was distributed uniformly in the cross section. Reaction condition I V A gave copolymer fabrics in which the initial shape of the fibrous cross section was unchanged, and the grafted polymer was concentrated in the outer layers of the cross section. Copolymer fabrics prepared by this method had improved flex and flat abrasion resistances and breaking strength as compared with the control fabric. However, tear resistance decreased compared with the control fabric, and flex abrasion resistance was lower than that for copolymer fabric prepared by method I V B . Method I V B could also be developed into a continuous process, so that the fabric would first be immersed in a solution of vinyl monomer and then irradiated. For all of the methods indicated there would be a reduction in the molecular weight of the cellulose as a result of oxidative depolymer ization. Type of Vinyl Monomer. As reported previously, one of the important factors in determining the morphology of fibrous cellulose copolymers is the type of vinyl monomer used (27, 28, 30). W e have also discussed the effects of type of monomer and chemical modification of cellulose on the thermal stress properties of copolymers. Similarly, the physical properties of the fibrous copolymers depend on these factors. For example, when the elastic recovery properties of fibrous cellulose copolymers, described in Table I, were examined, only copolymer D exhibited a change in these properties, as shown in Table V . At an elongation at break as high as 10%, changes in permanent set and delayed recovery were recorded. These changes were observed for copolymer D in both fiber and yarn forms (21, 24). In use as a textile, the elongation represents about the normal stress that would be applied to the fiber. The effects of type of vinyl monomer used on the properties of copolymer yarns are shown in Table V I . Compared with the cotton control, cellulose-poly (vinyl acetate) copolymers exhibited the largest increase in elongation at break and breaking toughness and the largest decrease in stiffness of the fibrous copolymers examined. Cellulosepolystyrene and cellulose—poly (methyl methacrylate) copolymers exhibited the largest decrease in breaking toughness. Cellulose—polyacrylo-


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MULTICOMPONENT POLYMER

SYSTEMS

nitrile copolymer exhibited only a small decrease in breaking toughness. A l l of the fibrous copolymers examined had lower stiffness values than the cotton control. Table V, Elastic Recovery Properties of Fibrous Cyanoethylated Cotton Cellulose (D.S. 0.7) (62%)-Polyacrylonitrile (38%) Copolymer


Elongation of

Recovery, % of Actual Elongation

Fibrous Copolymer, % of elongation at break

Purified Cotton Yarn

5 10 25 50 75

30 23 9 6 6

Immediate Recovery 34 29 10 8 7

34 24 9 5 4

5 10 25 50 75

49 58 66 46 29

Delayed Recovery 49 55 69 46 30

63 71 74 48 29

5 10 25 50 75

21 19 25 48 65

Permanent Set 17 16 21 46 63

3 5 17 47 67

Table VI.

Cyanoethylated Cotton Yarn

Cyanoethylated CottonPolyacrylonitrile Copolymer Yarn

Properties of Cotton Cellulose—Polyvinyl Copolymer Yarns a

Copolymer (Add-on)

Property

PAN Cotton PS

Multicomponent Polymer Systems

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