Computer Applications in Applied Polymer Science - American

9 UV Spectrophotometers as Detectors for Size Exclusion Chromatography of StyreneAcrylonitrile (SAN) Copolymers LUIS H. G A R C I A - R U B I O , A. E. H A M I E L E C , and J . F . M A C G R E G O R

Downloaded by JOHNS HOPKINS UNIV on October 8, 2013 | http://pubs.acs.org Publication Date: September 24, 1982 | doi: 10.1021/bk-1982-0197.ch009

1

McMaster University, Chemical Engineering Department, Hamilton, L 8 S 4 L 7 Ontario, Canada

The application of UV spectrophotometers to the analysis of styrene containing copolymers has been extensively reported in the literature. However, hypochromic effects and band shifts which result in deviations from Beer's Law have limited the use of UV detectors as mass or composition detectors in size exclusion chromatography applications. Deviations from Beer's Law for low conversion styrene-acrylonitrile copolymers in tetrahydrofuran have been experimentally investigated and compared with results previously reported in the literature. The behaviour of the extinction coefficient as a function of the copolymer composition is discussed in view of the information obtained from infrared and nuclear magnetic resonance measurements on the same polymers. As a result of this investigation, a quantitative correlation of the extinction coefficients of styrene-acrylonitrile copolymers with the length of the styrene sequences has been obtained which, in turn, allows for the use of UV spectrophotometers as sequence length detectors. Traditionally, spectrophotometers have been used in liquid and size exclusion chromatography applications as detectors for the measurement of concentrations and compositions. Three regions of the electromagnetic spectrum are generally used for mass detection. The ultraviolet (50-4000 A), the visible (4000-7000 A), and the infrared (7 ju.m-500 jbtm). In the visible region, polymer molecules generally do not absorb but quantitative changes in the refractive index (RI) of the solution are normally observed. Therefore, refractometers are used as standard SEC detectors. UV spectrophotometers have high sensitivity for aromatic and carbonyl bands and, because they monitor specific functional groups, are rarely influenced by environmental factors such as temperature pressure, etc. UV specificity is limited because not every compound absorbs in the UV region. It has been, nevertheless, successfully applied to strongly-absorbing solute bands in regions of high solvent transparency. Although infrared analysis of copolymers has been extensively used (1), it is with the development of new and powerful techniques (infrared lasers, Fourier Transformed Infrared (FTIR), Multiple Scan IR (MIR) etc.) that the infrared analysis of SEC effluents has become a useful technique. IR spectroscopy is more general than RI or UV since almost every organic molecule will show some form of absorption in the IR Region. Current address: Xerox Research Center of Canada, 2660 Speakman Drive, Mississauga, Ontario, L5K 2L1, Canada. 1

0097-6156/82/0197-0151$09.25/0 © 1982 American Chemical Society In Computer Applications in Applied Polymer Science; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

152

COMPUTER APPLICATIONS IN APPLIED POLYMER SCIENCE

In the use of spectrophotomers as mass detectors, two fundamental assumptions are made. i. The intensity of an absorption band is related to the concentration of the substance or functional group that absorbs the incident radiation. If the concentrations are small, Beer's Law is assumed valid (i.e. there exists a linear relationship between the absorption and the concentration). A


ii.

A i

= log

= e j j c,

1

(1)

The absorption spectrum of mixtures is in general additive, i.e. the sum of the individual spectra of the components.

A

X T

=

2

A

(2)

X j

i=1 where N is the number of components Similar assumptions are used for refractive index measurements. Anj =^dn

An

T

j

C|

N = 2 An i=1

{

(3)

(4)

It is obvious, that if the different functional groups present in a copolymer molecule absorb at different wavelengths, the composition and the polymer concentration can be readily obtained. Based on these premises UV, IR and RI are used for copolymer analysis. However, there have been indications over the years that the assumptions of linearity and additivity may not always be valid. Extensive studies on the refractive index of polymers in solution by Barrall, Cantow and Johnson (2), have shown that the specific refractive index increment (dn/dc) is a function of the molecular weight. The linearity of (dn/dc) with respect to composition has been verified for a number of systems mainly block copolymers (3, 4). It has not, however, been sufficiently tested for random copolymers (5) and, therefore, its validity in general cannot be assumed. Fluorescence studies by Vala, Hoebig and Rice (6) have shown that there is a difference between the absorption spectra of isotactic and atactic polystyrene in the region between 2,000 A and 2,500 A. The UV absorption behaviour of styrene copolymers has been investigated by O'Driscoll et al (7), Brussau and Stein (8) and Gallo and Russo (9). Deviations from Beer's Law of up to 60% are consistently reported. The set of results put forward by the above authors indicates that the assumptions of linearity (Eq. 1 and 3) and additivity (Eq. 2 and 4) should be verified for each particular polymer system. It is the purpose of this paper to discuss the use of

In Computer Applications in Applied Polymer Science; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

9.

GARCIO-RUBIO ET AL.

UV for Size Exclusion

153

Chromatography

spectrophotometers and refractometers as SEC detectors for the analysis of styrene acrylonitrile copolymers. Particular emphasis is placed on U.V. spectrophotometers from which it is shown, that average styrene sequence lengths can be obtained. Materials Styrene acrylonitrile copolymers were synthesized by bulk free radical polymerization at 60°C and 40°C over a wide range of initial monomer compositions (10). Copolymer compositions were determined by gas chromatography and verified by H NMR spectroscopy. The narrow MWD polystyrene standards (Pressure Chemicals) and the SAN copolymers were purified prior to the analysis by dissolution in THF and precipitation from absolute methanol.


1

Infrared and Near Infrared Spectroscopy The sensitivity of infrared measurements to composition has been studied by Wolfram and Grasselli (11). Significant shifts of the position of the C = N stretching band (-2,240 cm" ) of SAN copolymers of different composition are reported (Table I) but no significant changes in the intensity of the bands were observed. More recently, Mirabella (12) investigated the effect of the molecular weight on the infrared absorptivity of polystyrene standards at two frequencies 1,450 cm" and 1,493 cm"" . Some changes in absorptivity were observed at 1,493 cm" but were attributed to end group effects. The band at 1,450 cm" gave no indication of any effect due to differences in molecular weights. 1

1

1

1

1

TABLE i

Infrared Analysis of S A N Copolymer %st

yCsN(cm" )

Tg °C

0

2242

90

15

2240

104

60

2237

99

75

2236

97

1

A necessary step in the use of spectroscopy data is the assignment of absorption bands to specific functional groups or to properties known to be present in the molecules. Extensive encyclopaedic work has led to the identification of domains of absorption modes within the infrared region. Using these as reference (13), identification of some of the absorption bands for SAN copolymers in the fundamental vibrations region is possible. The assignment of polymer absorption bands on the near infrared region has not been, unfortunately, as extensively documented. However, a sufficient number of bands can be assigned, based on the works of Foster, Row and Griskey (14) and Crandall and Jagtap (15), to investigate the effect of composition on the absorption behaviour of SAN copolymers (16).


154


Experimental. IR measurements were done in a Beckman IR-5 spectrophotometer either on films or solutions. Films were prepared from 10% w/v or CH CI polymer solutions. Solution measurements were done in a 0.01 mm path length NaCI cell. THF, DMF or CHCI were the solvents normally used. Near infrared spectra were recorded from a Beckman DK-A spectrophotometer. 10 mm path length silica cells were used. Relatively high concentrations (5% w/v) of polymer in CHCI were used in this preliminary evaluation. Measurements were done at room temperature in all cases. Table II describes the copolymers used in the evaluation of the IR region. 2

2

3


3

Results and Discussion. The application of infrared spectroscopy to the analysis of SAN copolymers in solution has two limitations: the poor solubility of the polymers, which constrains the analysis to those regions where the solvents are transparent and the poor sensitivity of the infrared at low concentrations. This last problem is more severe when the characteristic absorption bands have medium or weak intensities (typically - C = N stretching). Of particular interest are the bands between 3 - 7 /xm, some of which can be detected in a variety of solvents and provide information on the AN/St ratio. The bands at 13 and 14 /xm are very strong and amenable to detection particularly in SEC applications. These bands contain information on the styrene concentration and the molecular structure (Hummel 1974). Figure 1 and Table III show the absorption behaviour of copolymers obtained at low conversions (homogeneous) and high conversion (heterogeneous). Clearly, the -C = N stretching band is sensitive to the changes in molecular structure induced by the change in the polymerization environment throughout the course of the reaction (i.e. composition, viscosity). The effect seems to be the greater, the smaller the acrylonitrile content. These observations are consistent with the near IR measurements at 2.83 ttm (Figure 3). In the case of the - C = N stretching, not only changes in intensity are observed with increasing acrylonitrile content, but also shifts in the position of the band can also be correlated with changes in composition as reported by Wolfram and Grasselli (H). Figure 2 shows the band shifts observed for low conversion copolymers. Other absorption bands up to 12.5 tim are not as sensitive to the acrylonitrile content and can be used for mass or composition detection. The absorption bands beyond 12.5 jum require more analysis before a definite statement can be made with regard to their sensitivity and information content. A summary of the near infrared absorption is presented in Figure 3 and Table IV. Refractive Index Measurements Refractive index and specific refractive index increments -+{p = dn/dc) of polymers in solution have been studied extensively in connection with light scattering measurements and size exclusion chromatography applications to polymer characterization for which refractometers are used as standard concentration detectors. Contrary to the observations made in the infrared region (12), refractive index increments have been shown to be a function of the molecular weight of the polymers (2) and, in some cases, of the copolymer composition (17). Therefore, the assumptions of linearity and additivity (Eq. 1 to 4) have to be verified for each particular polymer system. In the case of styrene/acrylonitrile copolymers, there is an additional uncertainty due to the



GARCIO-RUBIO ET AL.


Chromatography

Table II SAN Copolymers Used in Infrared and Near Infrared Studies Low Conversion U c o o ( / >

< o o o

.2

A 3.3 2

r

T

.2 Figure I.

O

V

•

.6

1

.8

IR absorption ratios for O , low conversion and copolymers.

I

I

1-0 high conversion



GARCIO-RUBIO ET AL.

Chromatography

Table III


Infrared Absorption

Sample R-01

Composition P. 2.38

2.78

Wave Length [im 3.52 3.45 4.28

4.49

6.28

6.72 6.92

09

.92

.126

.074

.561

.229

.184

.051

.10

.261 .250

08

.740

.126

.075

.594

.218

.181

.063

.084

.240 .251

07

.69

.130

.072

.602

.233

.175

.072

.084

.248 .272

06

.82

.128

.073

.585

.222

.174

.056

.094

.246 .255

05

.73

.131

.075

.602

.215

.184

.065

.086

.233 .252

04

.605

.137

.076

.640

.235

.184

.078

.074

.222 .248

03

.55

.132

.076

.561

.215

.181

.097

.066

.215 .252

09A

.95

.108

.066

.530

.231

.171

.034

.097

.256 .24

08A

.803

.128

.073

.572

.226

.174

.056

.094

.252 .252

07A

.743

.126

.073

.594

.213

.180

.061

.089

.233 .244

06A

.904

.126

.073

.606

.232

.177

.041

.105

.256 .258

05A

.801

.131

.076

.622

.223

.184

.056

.092

.256 .260

04A

.643

.133

.078

.648

.215

.184

.073

.084

.237 .256

09A

.95

.130

.077

.588

.229

.187

.041

.102

.264 .252

A - High Conversion Copolymers


157


158


Figure 2. IR shifts of the — C = N stretching band as function of composition P styrene mol fraction in the copolymer. Key: O, data of Grasselli (1973)and Q,this work. t


9.


GARCIO-RUBIO ET AL.

Chromatography


O

1

1 2

1

1 A

1

1

.6

1

1

.8

1

i

1-0

Figure 3. Near IR absorption as function of composition F -styrene mol fraction in the copolymer. Open symbols indicate low conversion and solid symbols indicate high conversion. Key to wavelength (fxm): 0 # , 2.8; A A , 2.27; 0 1.68; O Q, 1.76. ±


159

160



Table IV Near Infrared Absorption

Sample R-01

P

1

1.68

1.76

2.27

Wavelength (tim) 2.30 2.47 2.48

2.8

2.83

3.1

09

.92

.215

.093

.305

.229

.839

.870

.352

.357

.1871

08

.74

.208

.111

.276

.264

.745

.790

.456

.638

.229

07

.69

.184

.101

.252

.268

.703

.740

.438

.688

.229

06

.82

.222

.108

.305

.260

.824

.839

.532

.523

.208

05

.73

.211

.108

.280

.268

.756

.783

.495

.658

.237

04

.605

.200

.132

.252

.323

.721

.745

.585

.854

.292

03

.55

.185

.120

.240

.314

.757

.810

.683

.948

.280

09A

.95

.216

.0783

.326

.204

.886

.903

.288

.175

.164

08A

.803

.197

.086

.284

.233

.801

.818

.379

.171

.237

07A

.743

.197

.094

.277

.244

.780

.801

.456

.567

.215

06A

.904

.215

.084

.319

.222

.854

.893

.330

.301

.174

05A

.801

.211

.092

.294

.229

.796

.824

.403

.466

.208

04A

.643

.194

.108

.260

.269

.733

.757

.602

.770

.252

A - High Conversion Copolymers


9.

GARCIORUBIO ET AL.


161

Chromatography


limited solubility of acrylonitrile-rich copolymers in solvents which are transparent in the ultraviolet region. Experimental. The differential refractive indexes of polymer solutions were measured at 25°C with a Waters Scientific R-403 differential refractometer connected on-line with a size exclusion chromatograph. The refractometer was calibrated to refractive index units (Riu) with benzene/carbon tetrachloride solutions. The rationale behind using the refractometer on-line with the chromatograph is the elimination of impurities in the sample (water, residual monomer etc.) which affect the refractive index measurements particularly at low polymer concentrations and to calibrate the detectors at the flow conditions at which they were normally operated. Polymer solutions of several concentrations (0.015-0.0025 wt %) were injected repeatedly to verify the reproducibility of the measurements, which was typically An ± 0.5 x 1 0 ' for replicates on the same solutions. 6

Results and Discussion. The specific refractive index increments defined as: (5)

where An =

vc

+ kc

2

(6)

were estimated directly using equation 6 and measurements of refractive index differences of polymer solutions at several concentrations. Equation 6 was used instead of the standard linearized equation (An/c) in order to maintain the error structure in both An and the concentrations (10). The 95% confidence intervals for the values of v were, on the average: v ± 0.003 (Riu/g/cc) for replicates on the same sample. The pooled standard deviation for all samples is ± 2.0% or ± 0.004 (Riu/g/cc). A number of narrow polystyrene standards (Waters Scientific) were examined in order to quantify the dependence of the specific refractive index increments in THF on the molecular weight of the polymer. The results shown in Figure 4 agree quite well with those reported by Schulz and Baumann (18). The values of v increase slightly with molecular weight up to approximately 20,000 and then remain constant within the experimental error. Partial specific volumes were estimated using the equation



.0.10

.0.11

.0.12

.Q13

.0.U

.0.15

.0.16

.0.17

.0.18

.0.19

.0.20

L0.21

.0.22

'

i i

1

1

'I

10 Mil

1 1

dn dc

I

I

10

0

0^

I I I 1111

I I Mill

I

I

I

-4

I

I

.0-

I I

11

10

I I I I III

0

I

I

I

I

I

I I I

I

10

1 - °

-o-^-o-

1 1 I I 1 I1 1

1

1—|

4

Figure 4. Specific refractive index increments (dn/dc, data of Schultz and Bauman, 1968) and extinction coefficients (e X 10' , this work) of polystyrene standards as function of the molecular weight. Key: O, total copolymer measurements; O, data obtained with the SEC.

~>


| || |

9.


GARCIO-RUBIO ET AL.

163

Chromatography

where '1 -

V

1 n.,2

+2

n is the refractive index of the solvent, n the refractive index of the polymer, v is the specific volume of the polymer and v the partial specific volume of the polymer in solution. Equation 7 was derived independently by Machtle and Fisher (17) and Lorimer (19) assuming additivity of the specific refractivities (Equation 8) and the validity of the Lorenz-Lorenz equation (9) respectively. Q

v

1


1

V l

Hfzl= n2 + 2

2 vj (n -1)/(n i=1 2

2

+

2)

(9)

R total specific refractivity Pj mass fraction of component i V| total specific volume The results obtained with equation 7 are summarized in Table V. The specific volume of the polymer at infinite dilution is considerably smaller than the specific volume of the solid substance (v = 0.948 cm /g) in agreement with values reported elsewhere (4). Specific refractive indexes of SAN copolymers at several compositions were also measured in THF. Low conversion copolymers were initially investigated to correlate the effect of the composition on the refractive index increments (Fig. 5). All measured points lie on a straight line within the experimental error indicating that the assumption of additivity appears to be valid for both v and v 1

3

r

2

Ph. w .

i=1


(11)

164



Table V Refractive Index Increments and Extinction Coefficient of Polystyrene Standards

V

Sample

Mwx10"

3

Riu/g/cc

v

1

cc/g

€x10* A .cm u

2

g

PS02

.60

PS03

6.00

PS04

10.30

PS05

4

€xioAy.cm g

»

.14

•-

.1478

.1870

.89505

.143

.1454

19.85

.1955

.8766

-

.1495

PS06

51.00

.1961

.8753

PS08

173.00

.1942

.8794

»

411.00 PS12

PS16 Mean

4

.1512 --

.1497

.1538 .1574

867.00

.1970

.8734

.1537

1800.00

-

-

.1556

2145.00

-

-

.1608

7100.00

.1965

.8745

.1944

.8790

.1506

.1520

-.038 + .0135

-0.006 -0.018

-.071 +.067

-.044 + .046

.1590



Chromatography


GARCIO-RUBIO ET AL.

Figure 5. Specific refractive index increments of SAN copolymers. Key: O, low conversion; high conversion. MW of polystyrene standards indicated on right hand axis.


166


Polymers obtained at intermediate and high conversions with varying degrees of composition heterogeneity were also investigated (Fig. 5). No significant deviations were observed. The scatter present in the data could be attributed to errors associated with the composition and to differences in molecular weights. The specific refractive indexes v in equation 11 were estimated in a slightly different form: {

v

=

v

A

-{V,-P*)

(12)

1-P* i-r|

w

where Pj* and v* were chosen as the azeotrope's composition and refractive index increment. Equation 11 was transformed using Equation 12 to minimize the uncertainties associated with the extrapolation required to estimate the refractive index increment for polyacrylonitrile in THF. (Polyacrylonitrile is insoluble in THF). In the final estimation of v*, all the PS and SAN results were used. The estimated values are: = .1928 and v* = .1619 both with a 95% confidence region of ± 0.003. The value of the increment for polyacrylonitrile calculated from Equation 12 is P2 = .069 which is approximately half the value predicted by Equation 7 assuming that v^ = v Partial specific volumes for the copolymers were calculated using Equation 7. Unfortunately, the magnitude of the experimental error makes it difficult to draw conclusions on the behaviour of the volume of copolymer molecules in solution. The results obtained from refractive index measurements have been summarized in Figure 6 and Tables 6 and 7.


w

r

Ultraviolet Spectroscopy The qualitative and quantitative study of chromophores with ultraviolet light has undergone intensive development in classic organic chemistry and now constitutes routine analysis in many cases. (Chromophore groups are groups capable of undergoing electronic transitions between their different electron levels when energy corresponding to the ultraviolet wave lengths is applied to them. Typically these are conjugated systems such as dienes, phenyl groups etc.) Ultraviolet analysis applications to polymers have been centered, primarily, on polymers and copolymers containing styrene or styrene derivatives, where the group or chromophore responsible for the ultraviolet absorption is the phenyl ring in the styril units. Recent studies on the absorption behaviour of styrene containing polymers have shown significant deviations from the standard assumptions of linearity (Beer's Law) and additivity. These deviations have been attributed to the microstructure of the polymer chains and in some cases to the conformation of the molecules in solution. Clearly, if these observations can be verified for styrene/acrylonitrile copolymers, ultraviolet (UV) detectors could be used to monitor sequence length, tacticity or molecular conformation as functions of the molecular size in size exclusion chromatography applications. In view of the importance of these measurements, a review of the most relevant reports is included in this section. Literature Review. The early reports on the difference in absorption behaviour of polystyrenes in solution are due to Vala and Rice (6). These authors attributed the hypochromism (decrease in absorption intensity) observed in some of the absorption bands when the spectra of isotactic and atactic polystyrenes are compared to the conformation of the polymer molecules in solution. Hypochromic


GARCIO-RUBIO ET AL.


Chromatography

167


9.

Figure 6. Summary of refractive index increment data as function of the refractive index of the solvent (n ) O, Data from Baumann and Lange, Blanks and Shah, Kambe, and this work. See Ref. 10 for further explanation. 0



168


Table VI Refractive Index Increments of Low Conversion Copolymer Sample

P

1 w

v Riu-cm 9

3

^ cm /g 3

R-01-09 R-02-02 R-02-04 R-02-06 R-02-08 R-02-10 R-02-01A

.958 .873 .837 .799 .735 .690 .677

.1887 .1759 .1754 .1710 .1573 .1538 .1532

.8780 .8844 .8756 .8748 .8870 .8823 .8800

R-04-01 R-04-02 R-04-03 R-04-04 R-04-05 R-04-06 R-04-07 R-04-08 R-04-09

.550 .620 .670 .685 .724 .750 .788 .840 .896

.1377 .1513 .1547 .1578 .1555 .1622 .1686 .1734 .1825

.8789 .8686 .8749 .8722 .8879 .8805 .8770 .8808 .8764


GARCIO-RUBIO ET AL.


Chromatography

Table VII Summary of Refractive Index Increments and Partial Specific Volumes for Low and High Conversion Copolymers


Sample

P

1 w

v

~

Riu

cm3

9

, ™? 9

v

R-01-09 R-01-09-A

.958 .974

.1887 .1895

.8799 .8825

R-02-02 R-02-11 R-02-16 R-02-41 R-02-51

.873 .867 .876 .916 .941

.1759 .1763 .1756 .1868 .1850

.8844 .8819 .8859 .8725 .8833

R-02-04 R-02-17 R-02-32 R-02-47

.837 .832 .852 .879

.1754 .1688 .1749 .1747

.8756 .8886 .8808 .8863

R-02-06 R-02-13 R-02-43 R-02-53

.799 .784 .814 .814

.1710 .1678 .1699 .1705

.8748 .8776 .8813 .8780

R-02-08 R-02-14 R-02-19 R-02-49

.735 .731 .748 .746

.1573 .1560 .1577 .1586

.8870 .8887 .8897 .8872

R-02-10 R-02-15

.690 .701

.1538 .1546

.8823 .8835

R-02-01A SAN Std.

.677 .77

.1532 .1599

.8800 .8909



170


effects were also observed by O'Driscoll, Wertz and Husar (7) in the analysis of styrene methyl methacrylate (SMMA) copolymers. The hypochromicity was attributed to the interaction of phenyl groups on adjacent monomer units and correlated with the calculated fraction of dyads. Brussau and Stein (8) investigated the effect of the sequence length on the UV absorption of styrene containing copolymers such as styrene maleic anhydride (SMA), styrene butadiene (SB) and styrene acrylonitrile at several wave lengths in chloroform and THF. Their experimental results indicate that the response of the UV detector appears to be a function of the styrene sequences. Therefore, UV spectrophotometers were regarded as not being effective for concentration detection in chromatography applications to copolymer analysis. Stutzel, Miyamoto and Cantow (20) also investigated SMMA copolymers in THF and dioxane solutions at several temperatures. From their experimental results, they concluded that it is unlikely that the hypochromic effects are due to differences in conformation, but rather, that the differences in absorption are due to the sequential arrangement of the monomer units in the chain in agreement with O'Driscoll's and Brussau's conclusions. The average extinction coefficient for the copolymer was expressed as a function of the probabilities of the three styrene-centered triads: ^

=

f

SSS ^SSS

+

f

MSS ^MSS

+

f

1 3

MSM ^MSM

( )

where € is the observed average extinction coefficient. are the styrenecentered triad extinction coefficients and f are the styrene-centered normalized triad probabilities. Stutzel and Miyamoto suggest that the extinction coefficient ^sss k P P o a t e d by that of polystyrene, whereas € can be estimated from alternating copolymers. Stutzel's arguments have been strongly contradicted by the experimental evidence presented by Gallo and Russo (9), who studied the hypochromism of SMMA copolymers in solution as functions of the solvent's dielectric constant. Their results can be summarized as follows: • the hypochromic effects are present for copolymers and non-existent for mechanical mixtures of polystyrene and methyl methacrylate; • the percent hypochromism and the composition at which it appears are functions of the dielectric constant of the solvent; • solvents with high dielectric constant show smaller hypochromic effects. The above evidence indicates that hypochromism in SMMA copolymers is related to the conformation of the copolymer molecules in solution. Unfortunately, although it is known that the conformation of the molecules in solution is a function of the microstructure, the relationship between sequence length, tacticity and conformation has not been quantified. More recently, Gruber and Knell (21_) investigated the absorption behaviour of styrene/butyl acrylate and styrene/n-butyl acrylate copolymers at a wave length of 269 nm in chloroform. These authors concluded, in agreement with O'Driscoll, Brussau and Stutzel, that the differences observed between the expected behaviour of the extinction coefficients and the experimental values obtained could be explained in terms of the interaction between neighbouring aromatic nuclei (London forces). In other words, that the extinction coefficient of styrene containing copolymers is a function of the length of the styrene sequences in the copolymer. Whether the lengthening of the styrene sequences is directly responsible for the anomalous behaviour of styrene containing copolymers or whether it is due to a change in tacticity and/or conformation proceeding hand-in-hand with it, cannot be decided on the basis of the experimental evidence thus far reported in the literature. The general consensus favours the hypothesis that recognizes the jsj

c

a

n

e

a

r

xim

M S M


9.

GARCIORUBIO E T AL.

171

UV for Size Exclusion Chromatography

length of the styrene sequences as being the most important factor in explaining the observed deviations from Beer's Law.


Experimental. Absorbances of selected polymer and copolymer samples were measured with a Waters 440 ultraviolet absorbence detector equipped with a 254 nm filter, and a Beckman 25 ultraviolet/visible spectrophotometer. The Water's instrument was used on-line with the size exclusion chromatograph (16). The Beckman 25 was used to calibrate and standardize the Water's 440 UV detector. THF was used as standard solvent. The extinction coefficients were estimated from absorbance measurements at several polymer concentrations. The standard deviation for the absorbance measurements was typically ± .001 Au for replicates on the same sample. All measurements were done at room temperature. Results and Discussion. As a first step in the analysis of the absorption behaviour of SAN copolymers, the effects of tacticity and molecular weight on the extinction coefficients of well-characterized polystyrene samples were investigated. The spectra of isotactic and atactic polystyrenes are shown in Figure 7. The spectra are quite similar except for the bands at ~ 258 nm and ~ 262 nm which show differences in intensity i.e. the ratio A258/A262 increases for isotactic polystyrene. This ratio also increases with acrylonitrile content (Figure 8) indicating some degree of tacticity in agreement with the H NMR results (10, 22) and with the conclusions of Gallo and Russo. The absorption behaviour of narrow polystyrene standards as a function of the molecular weight is shown in Figure 4. The extinction coefficient increases slightly (~ 12%) over the molecular weight range 600-7.1 χ 10 . The reason for such increase is unknown. It can be speculated, however, that the increase in extinction coefficient with molecular weight is due to an increase in scattered light as the molecular dimensions approach the wavelength of the incident light. The extinction coefficients of selected polystyrene standards as functions of the molecular weight are reported in Table V. An average value of € = 0.152 χ 1 0 (Au.cm /mol) or on molar basis 6 ^ = 0.158 χ 1 0 (Au.cm /gms) was used for comparison with the extinction coefficient of SAN copolymers. The assumptions of linearity (Beer's Law) and additivity (Equations 1 and 2) can be stated for UV absorption as follows: 1

6

4

p s

2

Α

λ

6

= €

x

2

l c

(14)

and for multicomponents η h

= Σ Α. i=1

η λ

= |·

I € i=1

J X

-

C i

(15)

where Α.χ is the absorbance of the ith component at the wavelength λ, I is the cell's path length, € the extinction coefficient of the ith component and Cj is the concentration of the ith component. For copolymers (n = 2) equation 15 can be written as: j X


172



Figure 7.

UV spectra of a, atactic and i, isotactic polystyrenes in THF wavelength (nm).



9.


GARCIO-RUBIO E T AL.

173

Chromatography

.20

J8J .16

J

©o © © ©

J

.12 J

.10

i

1—i—i .2

Figure 8.

A

1 — i — i — r

1 .6

.8

1.

Molar extinction coefficients of low conversion SAN copolymers in THF at 25°C, 254 nm.


174


A

X

= I c ^ x P ,

+

C^d-P,))

(16)

where c is the copolymer concentration (gmol/cm ) and P is the mole fraction of monomer 1 in the copolymer. If only one of the monomers absorb at a given wave length, equation 16 can be simplified to 3


A\-i-c-e

l X

p

1

d7)

1

a standard assumption made in the UV analysis of styrene containing copolymers is that, the only chromophore present in the copolymer molecules is the phenyl group in the styril units (see references cited in Literature Review). This assumption is justified on the basis that the nitrile group absorbs significantly only at wave lengths

Computer Applications in Applied Polymer Science - American

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