Pulsed Laser Experiments Directed Toward the Detailed Study of Free

Chapter 6

Pulsed Laser Experiments Directed Toward the Detailed Study of Free-Radical Polymerizations Sabine Beuermann and Michael Buback

1

Downloaded by UNIV OF IOWA on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch006

Institut für Physikalische Chemie der Georg-August-Universität, Tammannstrasse 6, D-37077 Göttingen, Germany

Pulsed-laser polymerization (PLP) in conjunction with either molecular weight analysis of the polymeric product or time-resolved measurement of monomer conversion induced by a single pulse or by pulse sequences allows for the reliable determination of rate coefficients in free-radical polymerization. This article is primarily concerned with the application of P L P methods toward measuring propagation rate coefficients, k , and chain-length averaged termination rate coefficients, , as a function of temperature and partly up to high pressure. Termination rate coefficients may significantly vary during the course of a free-radical bulk polymerization. P L P experiments allow to study these changes. They also provide access to the investigation of a chain-length dependence of k . Moreover, chain transfer rates may be derived from P L P experiments. p

t

t

Over the past ten years, since the introduction of pulsed laser techniques into the detailed study of free-radical polymerization, a considerable amount of accurate rate coefficient data has become available. The principal types of pulsed laser polymerization (PLP) experiments are illustrated in Figure 1. Among them, the P L P S E C (size exclusion chromatography) technique, pioneered by Olaj and coworkers {1,2). is of primary importance. A n evenly spaced sequence of laser pulses is applied onto a monomer/photoinitiator system and a small initial monomer conversion, typically of one per cent, is reached. The almost instantaneous production of free radicals by each laser pulse causes an enhanced termination probability for radicals from the preceding pulse(s). This situation gives rise to a characteristic structure of the molecular weight distribution ( M W D ) , as is illustrated by a simulated (see below) polymer size distribution in Figure l a . where the weight fraction w is plotted vs. the logarithm of molecular weight, \og\oM. A s will be shown in Section I. the propagation rate coefficient, k . may be directly obtained from the structured M W D . v

'Corresponding author.

84

© 1998 American Chemical Society

In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

85

PLP - SEC experiment —* k

a


0

b

0.1

0.2 Ms)

0.3

SP - PLP experiment —• kjk

11 s C PS - PLP experiment —•

Figure I:

(Olaj, Bitai, Hinkelmann 1987)

3,5

4

4,5 log M

5

12

5,5

1 0

(Buback, Hippler, Schweer, Vôgele 1986)

3

îIs (Buback, Huckestein. Leinhos 1987)

4

Pulsed laser methods used to evaluate rate coefficients for free-radical polymerization.

The application of the P L P - S E C technique is restricted to studies at low monomer conversions up to a few per cent. Information about kinetics during the course of a polymerization is available from the single pulse (SP)-PLP experiment (Figure lb) which may be performed at any time during polymerization (5). The monomer conversion induced by the laser pulse, which usually has a width of about 20 ns, is


86

recorded by online vibrational spectroscopic analysis with time resolution in the microand millisecond range. From the measured conversion-time trace the ratio of rate coefficients. k /k , becomes available for the rather narrow conversion interval of the experiment, extending typically over no more than 0.1 to 0.5 per cent. With k from P L P - S E C analysis, the S P - P L P experiment thus provides direct access to a chainlength averaged termination rate coefficient, , for a well-defined narrow conversion region. The S P - P L P and P S - P L P experiments are carried out such that the entire sample is irradiated and that significant gradients in free-radical concentration are avoided (3-5). {

r

p

The S P - P L P experiment may be performed at several stages during a polymerization in order to map out the conversion dependence of over a wide range. A n important point to note is that the S P - P L P experiment in addition to providing (for each of the narrow conversion intervals where an experiment has been carried out) may also be used to investigate chain-length dependent £,(/,/) for a given conversion (interval). This is easily understood from the r.h.s. plot in Figure l b . The measured change in conversion, AU, with the time t (after applying the laser pulse) is brought upon by small radicals at low t, by medium-sized free-radicals at intermediate t and by large radicals after extended times t. The size of the free radicals which are created by the laser pulse at t = 0 linearly increases with time t unless chain transfer events come into play. As a consequence, the S P - P L P experiment provides a unique access to measuring k (iJ) where / characterizes chain-length. The sizes / for each pair of terminating radicals in an S P - P L P experiment are almost identical as both species are generated at the same time. k (ij) characterizing termination of two radicals differing in size is not easily accessible from this type of experiment. If not stated otherwise, the subsequent discussion addresses the chain-length averaged termination rate coefficient, , which will be referred to as k .


{

t

t

t

x

The S P - P L P experiment, in order to achieve a reasonable signal to noise of the AU vs. t trace (Figure lb), requires a considerable propagation rate of the monomer under investigation. It is for this reason that S P - P L P studies until now have been restricted to high Ap monomers, such as acrylic esters and to ethene at high temperature. For the slowly propagating monomers, e.g. methacrylic esters and styrene, where conversion per pulse is small, the pulse sequence (PS)-PLP procedure has been derived (4,5). The monomer conversion induced by a precisely known number of laser pulses is measured. In order to introduce some kind of time resolution within the experiment, the technique is mostly applied with pulse repetition rate alternating between subsequent pulse packages. In situations where both k and k remain constant over extended initial ranges of monomer conversion, as found for the methacrylates and for styrene. kyjk is directly obtained from such an alternating P S - P L P experiment as depicted in Figure 1c. p

x

{

Within the subsequent text, applications of these three basic types of P L P experiments (Figs, la-c) will be demonstrated. Section I illustrates the potential of P L P - S E C procedures mainly for k analysis, but also for the measurement of chain transfer and of termination at low conversion and thus at low levels of polymer concentration. Section II primarily addresses the investigation of conversion dependent *, via S P - P L P and P S - P L P techniques. p


87

I. P L P - S E C Experiments

The principle underlying the measurement of k via P L P - S E C experiments has already been outlined in the previous section. The method has been extensively used during recent years and detailed discussions of the procedure are given in the literature(6,7) including publications by the I U P A C - W o r k i n g Party "Modeling of Polymerization Kinetics and Processes" (8,9). The S E C trace shown on the r.h.s. of Figure l a is a simulated curve which has been obtained by P R E D I C I simulation (10) for a styrene P L P at 7 0 C (7). The P R E D I C I program allows for a simulation of the full distribution of free-radical and polymer chain lengths (10). The structure of S E C traces from real experiments is less pronounced, in most cases only two maxima or one maximum with a shoulder are observed. This is a consequence of imperfections of the experiment, such as polymerization that is not induced by pulsed laser light, chain transfer activity, gradients of free-radical concentration in space and in time and, most importantly, axial broadening during gel permeation chromatography ( G P C , S E C ) . ?


P

The propagation rate coefficient k is derived from a characteristic degree of polymerization. LQ. which is directly available from the S E C trace, via eq 1 : p

where CM is the monomer concentration and ν the laser pulse repetition rate. There has been quite some debate about whether to identify LQ with the location of the point of inflection on the low molecular weight side of the maximum (according to the suggestion by Olaj et al. (1,2) or whether to read LQ from the peak maximum position. This latter procedure has been advocated by Sarnecki and Schweer ( / / ) . A detailed study (7). on P L P - S E C curves, simulated (via P R E D I C I ) for a wide range of P L P conditions, revealed that under conditions of c ° , the free-radical concentration generated by a single laser pulse, being not too high, the determination of k from the inflection point position constitutes the far more reliable general procedure. In addition, at low and moderately high CR° values, the internal consistency check of observing a second or even third point of inflection at degrees of polymerization around 2 Ln and 3 LQ may be performed. R

P

Figure 2 shows an example of data from this study (7) of simulated styrene P L P S E C curves. The pulse laser induced free-radical concentration c ° has been significantly varied. The k value introduced into this particular P R E D I C I simulation is 499 L m o l ' s ' . A s can be seen, analysis (via eq 1) based on point of inflection positions yields this value back within ± 2 per cent. Moreover, the analysis is almost insensitive toward the type of M W D : number distribution. f(M), weight distribution, \\(M). or logarithmic weight (or S E C ) distribution, M'(logmA/), from which the point of inflection is derived. At low levels of CR°, the maximum position (from all these M W D s ) significantly fails to reproduce the true k value (Figure 2). On the other hand, at fairly high values of c ° , situations may be reached where the peak maximum position provides a better measure for LQ (Figure 2). R

p

p

R


88

pulse laser induced free-radical concentration c Figure 2:

0 R

/ mol L

R

p

Polymer Handbook (1989)

Γ

1

Influence of free-radical concentration c ° on the accuracy of k determination by P L P - S E C ; data obtained from simulation via P R E D I C I . (Reproduced with permission from ref. 7. Copyright 1996 from Hiithig & Wepf Publishers, Zug, Switzerland.)

IUPAC(1995)

ο CD

CCS οοο ο ο

oo

(D


580^

< 0 jo° 2ΓI 2.6

ο 0

I I 1I 1I1 I. I I I I I I I I I I I I II ιI ιI ιI ι I II ιI ιI ιI ιI II ιI ι I ιI ι I ι .Γ. .I 1I I 1I I. .1 1I I1 I1I .I .I . I 1 I I 1I I. II I. II I1I I I. I1 I. I1 I 1I 1I 1I 1 I I 1I 1 I1

2.8

3

3.2

3.4

3.6

3.8

2.8

3

ΙΟΟΟΓ'/Κ-'

Figure 3:

3.2

3.4

3.6

3.8

4

ΙΟΟΟΓ'/Κ-'

Variation of propagation rate coefficient k with temperature for styrene bulk polymerization at ambient pressure. (Partly reproduced from ref. 9 with permission. Copyright 1996 from Hiithig & Wepf Publishers. Zug, Switzerland.) p

It is beyond the scope of the present paper several other experimental parameters, e.g. of and of axial broadening in S E C analysis, on interested reader is referred to ref. 7. If the

to review in any detail the influence of gradients in free-radical concentration the quality of k measurement. The recommendations put forward by the p


1.

89

I U P A C Working Party (9) are considered and the internal consistency checks are performed, the P L P - S E C experiment turns out to be very robust. A s the M W D of polymer material produced by P L P depends in a rather complex way on experimental parameters, it is certainly useful to accompany P L P studies by simulation. Figure 3 demonstrates the enormous impact of P L P - S E C on the reliable estimate of k for styrene homopolymerization over a wide temperature range at ambient pressure. The data on the l.h.s. of Figure 3 are the entries from the Polymer Handbook ( 12). The data on the r.h.s. are the results from independent P L P - S E C experiments performed in eight laboratories. r


8.0

4.5

J

1

1

2.75

1

3.00

3.25 3

1

1

3.50

3.75

1 0 · Γ ' / Κ" Figure 4:

1 4.00

1

Temperature dependence of the propagation rate coefficient k at ambient pressure for methyl methacrylate (circles), butyl methacrylate (squares) and dodecyl methacrylate (triangles); open symbols are data from ref. 13 and full symbols from ref. 14. p

Very satisfactory agreement is also obtained for k data of the methacrylate family with the experiments, however, coming from a smaller number of groups. Figure 4 shows ambient pressure k data reported by Hutchinson et al. (13) and by our group (14) for methyl methacrylate ( M M A ) , butyl methacrylate ( B M A ) , and dodecyl methacrylate ( D M A ) . Due to the high quality of k data from P L P - S E C , finer details such as the slight increase of k with the size of the ester group are clearly detected from the data. The temperature dependence of k for these methacrylates (at ambient pressure) is given by the relations: p

p

p

p

p

M M A (15):

l n [ * / (L · m o l "

1

B M A (14):

ln[* / ( L - m o l "

1

p

p

·s

- 1

)j= 14.79 - 2686(7' / Κ )

- 1

= 14.79-2638(Γ/ K)"

1


90

- 1

D M A (14):

ln[* / ( L m o l

·s

styrene (9):

| [ * / ( L · mol" · s

p

1

Η

- 1

- 1

Ρ

)] = 14.71 - 2 5 3 6 ( Γ / Κ)

)] = 17.57 - 3 9 1 0 ( Γ / Κ )

The Arrhenius relations for B M A and D M A are constructed from the combined data sets of Hutchinson et al. (13) and Buback et al. {14). In Table I k values of M M A . B M A . D M A . and styrene are listed for 30 °C and ambient pressure. p


monomer

Table 1:

1

^/(Lmol's )

styrene

106

methyl methacrylate

373

butyl methacrylate

439

dodecyl methacrylate

567

Propagation rate coefficients k at 30 °C and ambient pressure as calculated from the Arrhenius relations given above. p

5.00 4.75 4.50 -

DA (+15 °C)

4.25 4.00

M A (+15 °C)

3.753.503.25-

DMA (+30 °C)

3.00BMA (+30 °C)

2.75 2.50

M M A (+30 °C)

2.25 2.00 0

500

1000

1500

2000

ρ I bar Figure 5:

Pressure dependence of the propagation rate coefficient k for acrylates studied at 15°C and for methacrylates at 30°C. Monomers are as follows: methyl acrylate ( M A ) , dodecyl acrylate ( D A ) , methyl methacrylate ( M M A ) , butyl methacrylate ( B M A ) and dodecyl methacrylate ( D M A ) ; methacrylate data from ref. 14; acrylate data from ref. 16. p


91

In addition. P L P - S E C studies have been performed at high pressures. Such experiments are attractive, because only the P L P part of the experiment needs to be carried out under pressure. A s an example of such investigations, the pressure dependence of k for M M A . B M A . and D M A is plotted together with data for methyl acrylate ( M A ) and dodecyl acrylate (DA) (16) in Figure 5. p

As with the methacrylates. k is seen to increase with ester size for the acrylate family. The propagation rate of acrylates is significantly above k of the methacrylates. The clear difference in k suggests a "family-type" behaviour for acrylic acid alkyl esters and methacrylic acid alkyl esters, respectively. Also significant differences are seen between the activation energies, 15 and 17 kJ-mol" for the acrylates and 21 to 23 kJ-mol for the methacrylates, and between the activation volumes A V ( / r ) = R-T-id \nkp/d p) > -10 and -12 cnV-mol" for the acrylates and -16 to -17 c m m o l for the methacrylates. p

p

p

1

1

p


1

3

1

T

P L P - S E C investigations of acrylate k in extended ranges of temperature and pressure are less easily performed. As a consequence of the high propagation rates, molar masses may be controlled by chain transfer processes whereas k measurement via P L P - S E C requires that termination resulting from laser pulses is the major chainstopping event. The associated problems may be partly circumvented by using high pulse repetition rates. p

p

In situations where propagation is not too fast, by a special choice of the laser pulse pattern, k and chain transfer rate coefficients k may be derived from a single experiment. Figure 6 illustrates a suitable pulse sequence. P

tT

I

ι , 4

,

,

ι

ι

,

'

5

6

7

Τ

1

'

8

log M 1 ( )

Figure 6:

Laser pulse sequence as used for the determination of the propagation rate coefficient k and of the transfer rate coefficient k from a single P L P - S E C experiment. For further details see text. (Reproduced with permission from ref. 18. Copyright 1996 from Hiithig & Wepf Publishers. Zug. Switzerland.) p

u

1

Three laser pulses separated by a time interval of t = v" are followed by an extended dark time t

Pulsed Laser Experiments Directed Toward the Detailed Study of Free

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