Copolymers, Polyblends, and Composites

17 Grafting Kinetics in the Case of ABS G. RIESS and J. L. LOCATELLI

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch017

Ecole Supérieure de C h i m i e , 3 rue A . Werner, 68093 Mulhouse Cédex, France

During copolymerization of styrene (S) and acrylonitrile (AN) in the presence of polybutadiene, graft copolymer and free SAN were formed. After separation by a reversible crosslinking technique, the AN content and the molecular weight of grafted SAN and free SAN were determined. The difference in composition of these two species results from the preferential solvation of polybutadiene by styrene. The molecular weight of grafted SAN was higher than that of free SAN. This occurred even before macroscopic phase separation, and it can be attributed to the lower termination rate in the polybutadiene medium and to the preferential solvation of polybutadiene by the peroxide. Polymerization rate and grafting efficiency are given as functions of the 1,2-vinyl content and the type of initiator.

I

n rubber-modified polymers like high impact polystyrene or acrylonitrilebutadiene-styrene ( A B S ) resins, the toughening effect of the dispersed rubber particles appears only in the presence of block or graft copolymers. These copolymers regulate the particle size of the rubber dispersion and achieve adhesion of the two phases. Hence, graft copolymers are of practical importance in polymer alloys. W e achieved a systematic kinetic study of A B S . A B S resins, w h i c h are formed by copolymerization of styrene (S) and acrylonitrile ( A N ) i n the pres ence of polybutadiene ( P B ) , consist essentially of a mixture of S A N graft copolymer on P B and ungrafted S A N (styrene-co-acrylonitrile). The grafting kinetics and characteristics of the graft copolymer were studied i n relation to the preferential solvation effects as functions of different variables: type and concentration of P B , type and concentration of initiator, monomer concentration, conversion degree, etc. Separation and Characterization of the Graft Copolymer The grafting reaction occurs in benzene solution. Since the common separation techniques (e.g., selective extraction and fractional precipitation) were difficult to apply and were not always reproducible in separating small amounts of graft copolymers, we developed a new separation method based on reversible crosslinking. Formation of reversible gels greatly enhanced the solubility difference between the grafted and non-grafted species so that separation by solubility difference became very easy ( I , 2). 186

Platzer; Copolymers, Polyblends, and Composites Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

17.

RiESS A N D L O C A T E L L I

187

Grafting Kinetics of ABS


Reversible crosslinking of the polymer backbone, e.g. polybutadiene, a n d the graft copolymer was effected b y fixing C O O N a groups selectively o n the P B part (3, 4). T h e dipole-dipole interaction between C O O N a groups i n nonpolar solvents like benzene l e d to practically complete crosslinking of the polymers that contained C O O N a groups, e.g. the graft copolymer a n d even tually the polybutadiene that h a d not been grafted. T h e non-grafted S A N , however, remained soluble and was easily removed b y separation of the t w o phases: soluble and gel. Reversible crosslinking can be represented schematically: Polar Medium

Nonpolar Medium

(benzene-CH OH) 3

COONr

COONa

elimination of CHsOH

(benzene)

addition of C H O H

COONa

COONa

3

soluble form

gel

The reversibility of the crosslinking made it possible to change easily from a crosslinked state to a soluble one and vice versa. Rupture of the cross links was simply effected by adding a solvating agent, like C H O H , for the C O O N a groups; i n this w a y the soluble chains (non-grafted S A N ) retained in the crosslinked network were released. After a new crosslinking, the soluble chains, that were now accessible, were separated from the P B fraction that con tained the graft copolymer. This cycle of gel and soluble forms was repeated until pure graft copolymer was obtained. T h e use of C - l a b e l e d S A N as tracer revealed that, after three successive separation steps, the remaining A B S was free of non-grafted S A N . The weight a n d S A N content of the different fractions are noted o n the diagram depicting the purification procedure. Methanol was eliminated from this system at room temperature by simple azeotropic distillation under light vacuum. 3

14

crude A B S G

0

S A X content, 78.8% soluble Si weight fraction, 29% S A N content, 98%

weight fraction, 8 % S A N content, 93.3 % S

3

weight fraction, 2.5% S A X content, 87%

weight fraction, 60% S A X content, 53.6%


188

COPOLYMERS,

POLYBLENDS,

AND COMPOSITES

After separation of the pure graft copolymer, its S A N content was deter mined. I n order to compare the characteristics of grafted and non-grafted S A N , the P B was selectively oxidized ( 5 ) . T h e molecular weight and the acrylonitrile content of the graft could then be determined.


Characteristics of the Graft Copolymer Composition of the Grafts—Preferential Solvation. A systematic study of different parameters has revealed that there could be an important difference in composition between grafted and non-grafted S A N . Especially at low con version, this difference i n A N content could be m u c h greater than 4 % , w i t h resultant incompatibility of the t w o types of S A N prepared i n the same batch (6). Some of the data, which have been published elsewhere ( 7 ) , are presented in Table I. T h e difference i n acrylonitrile content of the grafted and the non-grafted S A N is apparent. Table I.

Experiment

Composition of Grafted and Non-Grafted S A N

Initiator*

BzoOo ΑΙ Β Ν BzoOo Βζ ()·> Bz 0 ΑΙΒΧ ΑΙΒΧ ΑΙΒΧ Βζ ()

8 7 3 9-1 2 1 5 6 4

2

2

2

2

2

Conversion Degree,

%

7.35 10.6 11.5 21.5 23.6 33.0 43.8 43.8 49.5

AN in Non-Grafted SAN, %

24.0 23.70 24.20 24.70 24.45 24.35 24.05 23.85 24.10

a

AN in Grafted SAN, %

R

22.0 22.2 22.5 23.2 23.3 22.2 23.15 22.8 23.1

1.090 1.075 1.075 1.060 1.055 1.095 1.035 1.045 1.040

c

Experimental conditions: P B concentration, 54 g/l; and monomer concentration (styrene + acrylonitrile), 20 wt % in solution. Bz 0 = benzoyl peroxide, A I B N = azodi(isobutyronitrile), [ B z 0 ] = (1.45-3.1) X 10~ moles/l, and [ A I B N ] = (0.55-1.6) Χ 10~ moles/l. _ % A N in non-grafted S A N % A N in grafted S A N ' a

b

2

2

2

2

3

3

R

_

B y different techniques, especially b y viscosimetry, it can be shown that the higher styrene content of the grafts results from the preferential solvation of P B b y styrene. A c c o r d i n g to Dondos and Benoit (8), the preferential solva tion can be expressed b y λ', which is the excess volume of styrene near or i n the polymer coil per gram of P B . Therefore, i n the close vicinity of the P B , the concentration of styrene is higher than its concentration far from any macromolecule. This effect correlated with the interaction parameters, χ, and w i t h the solubility parameters, 8, of the different reagents (9, 10). F o r example, i n Figure 1, the evolution of λ', w h i c h is characteristic of the preferential solva tion, and the evolution of AC, w h i c h is the difference i n composition between grafted and non-grafted S A N , are plotted as functions of the acrylonitrile volume fraction (11). These observations also confirmed the hypothesis of two independent polymerizations even before visible phase separation, the one producing grafted S A N and the other non-grafted S A N . It was also demonstrated that at low conversion internal free S A N is formed i n the polymer coil; this non-grafted S A N therefore had the same composition as the grafted S A N (12).



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RiEss A N D L O C A T E L L I

189


M o l e c u l a r Weights. Furthermore, it appeared that the molecular weight of grafted S A N was systematically higher than that of free S A N , regardless of whether B z 0 or A I B N was initiator (see Table I I ) . The study of this molecular weight difference as a function of other parameters (e.g., polybutadiene and monomer) corroborated the fact that even before visible phase separation there were two independent polymerization reactions: one i n the P B medium that produced grafted S A N , the other that produced free S A N . T h e difference in molecular weight results from this 2

2

Table I I .

Run B-l 8 3 B-2 7 A-l

M o l e c u l a r W e i g h t of Grafted a n d Non-Grafted S A N " Initiator I

[/], mole/I X 10*

M Grafted SAN

Non-Grafted SAN

Bz 0,

1.0 1.45 2.0 2.35 0.55 1.35

138,000 129,000 109,000 108,000 166,000 125,000

85,000 82,500 80,000 80,000 85,000 79,500

2

AIBN

W

° Experimental conditions: conversion degree, 10%; concentration P B , 54 g / l ; total monomer concentration, 2.05 moles/l; and temperature, 7 0 ° C .


190

COPOLYMERS,

Table III.

POLYBLENDS, AND COMPOSITES

Preferential Solvation of P B b y B z Q 2

[PB],

g/200 ml

2A 2B 2E 2F

[SAN],

6.65 4.45 6.65 4.45

2

X 10*

X

1.44 1.46 1.35 1.65

6.65 4.45 2.22 6.65

S

&

2.28 2.24 2.30 3.18

0.63 0.65 0.59 0.52

Experimental conditions: polybutadiene: Cariflex B R 1202, Λ/ = 100,000; initial B z 0 concentration, 1.3 χ 1 0 mole/1; and S A N : random copolymer of azeotropic composition, 24% Α Ν , Λ/η = 100,000. a


2

in SAN phase, mole/ml

in PB phase, mole/ml

g1200 ml

a

[Bz 0 ]

{BZ2O2]

Experiment

2

η

2

2

- 3

b s

=

[ 2 Û 2 ] in P B B z

[ B z 0 ] in S A N ' 2

2

heterogeneous polymerization with a lower termination rate i n the P B medium where the grafted S A N was produced. H o w e v e r this effect was partially counterbalanced b y an increase i n the initiator concentration in the P B medium because of preferential solvation of P B b y initiators like B z Q (12). 2

2

Kinetic Study Preferential Solvation b y Peroxides. I n a way similar to that used for the study of preferential solvation of P B b y styrene, w e also examined that solvation effect b y different types of peroxides. T h e amount of peroxide i n the P B was maximum when their solubility parameters were the same. F u r thermore, the solvation effect was increased by adding to the system nonsolvents of P B and of the initiator. Data on the preferential solvation of P B b y B z 0 as w e l l as data on the peroxide surconcentration, S, are presented i n Table III. Rate of Polymerization—Retardation Effect. B y studying the rate of polymerization i n A B S formation, it was possible to demonstrate the impor tance of the concentration a n d the structure of P B , especially its 1,2-vinyl content (see Table I V ) . B y systematic study, w e correlated the retardation effect of P B w i t h the 1,2-vinyl content, the efficiency of the free radicals, and the preferential solva tion of P B b y the peroxides. This retardation effect, w h i c h disappeared i n the presence of P B with high 1,2-vinyl content, was the consequence of two coincident phenomena: 2

Table I V .

2

Effect of 1,2-Vinyl Content of Polybutadiene on Rate of P o l y m e r i z a t i o n 0

Run

A 17 20 16 18 19

1,2-Vinyl Content,

Rp I Rpo ^

%

AIBN

without P B 0 4 18.5 70 95

BZ202

1 0.86 0.63 0.705 1.23 1.45

1 0.84 0.68 1.20 1.45

"Experimental conditions: polybutadiene concentration, 26.9 g/1; temperature, 7 0 ° C ; and monomer concentration, 1.90 moles/l. Rp/Rpo = degree of conversion in presence of polybutadiene/degree of conversion in absence of polybutadiene ; [AIBN] = 1.1 χ 10~ mole/1; and [ B z 0 ] = 1.6 X 10~ mole/1. 6

3

2

2

3



17.

RiEss A N D L O C A T E L L I


191

(a) a non-homogeneous distribution of the peroxide and therefore of the free radicals w i t h a higher concentration i n the rubber phase, w h i c h resulted from the preferential solvation of P B b y the initiator, a n d (b) a reduced efficiency of the radicals i n the P B medium b y formation of more stable macroradicals and b y a cage effect resulting from an increased viscosity i n this m e d i u m . Grafting Degree. T h e variation i n degree of grafting was correlated w i t h the 1,2-vinyl content of P B , the monomer concentration (effect of preferential solvation b y the monomer), the type and the concentration of initiator (prefer ential solvation b y initiators), and the degree of conversion. T h e effect of the structure of P B on the grafting degree is tabulated i n Table V . Table V .

Effect of Polybutadiene Structure on Degree of Grafting"

Type of PB

1,2-Vinyl Content, % '

E Q B 6 alt, Cariflex B R 1220 J . L . 15000 E G 62/4 P V B 40

0 4 18.5 70 95

Grafting Degree

b

AIBN

Bz 0

8.5 17.2 11.5 29.0 46.5

23.7 — 21.0 34.4 44.8

2

2

Experimental conditions: polybutadiene concentration, 25 g/1; and monomer concentration, 2.05 moles/l. Grafting degree = grafted S A N / t o t a l S A N ; [AIBN] = 1.1 X 10~ mole/1; and [Bz 0 ] = 1.6 Χ Ι Ο " mole/1. a

3

b

2

2

3

The lower grafting degree i n the presence of A I B N , compared w i t h that obtained with B z 0 , resulted from the greater stability of the primary free radicals; stable radicals cannot transfer hydrogen atoms i n allylic position o n I, 4-c£s-polybutadiene. I n the presence of a 1,2-vinyl structure, the tertiary hydrogen atoms were easier to remove, and in this way the A I B N could initiate grafting. Consequently, the presence of 1,2-vinyl structures increased the grafting efficiency (13, 14, 15). A mechanism based on two simultaneous polymerizations can be proposed. Such a mechanism w o u l d account for the preferential solvation and the hetero geneity of the system (4, 13). 2

Literature

2

Cited

1. Llauro-Darricades, M . F., Banderet, Α., Riess, G., Makromol. Chem. (1973) 174, 105. 2. Ibid. (1973) 174, 117. 3. Locatelli, J. L., Riess, G., Eur. Polym. J. (1974) 10, 545. 4. Locatelli, J. L., Thesis, Mulhouse, 1973. 5. Locatelli, J. L., Riess, G., Angew. Makromol. Chem. (1972) 26, 117. 6. Molau, G. E . , J. Polym. Sci. Part A (1965) 3, 4235. 7. Locatelli, J. L . , Riess, G., Angew. Makromol. Chem. (1972) 27, 201. 8. Dondos, Α., Benoit, H . , Makromol. Chem. (1970) 133, 119. 9. Locatelli, J. L., Riess, G., Angew. Makromol. Chem. (1973) 32, 101. 10. Locatelli, J. L., Riess, G., J. Polym. Sci. Part A-1 (1973) 11, 3309. II. Ibid., Part Β (1973) 11, 257. 12. Locatelli, J. L., Riess, G., Makromol. Chem. (1974) 175, 3523. 13. Locatelli, J. L . , Riess, G., Angew. Makromol. Chem. (1973) 28, 161. 14. Ibid. (1973) 32, 117. 15. Ibid. (1974) 35, 57. RECEIVED May 7, 1974. This work was supported by the Société Nationale des Pétroles d'Aquitaine.


Copolymers, Polyblends, and Composites

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