Photoinitiated Polymerization - American Chemical Society

Chapter 36

Downloaded by MONASH UNIV on January 9, 2017 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch036

Real-Time Fluorescence for Determining the Relative Sensitivities of Reactive and Non-Reactive Fluorescent Probes Wolter F. Jager and Otto van den Berg Department of Polymer Materials and Polymer Engineering, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands

The relative sensitivities of structurally similar nitrostilbene based fluorescent probes, bearing substituents of different sizes and reactivities, were determined during the photoinitiated polymerization of HEXDMA by real-time fluorescence spectroscopy. Reactive probes are significantly more sensitive than corresponding non-reactive probes, up to conversions of 78-87%, and the reactivity of the substituent, has a very pronounced effect on the probe sensitivity. Increasing the probe size resulted in a small decrease in probe sensitivity at all stages of the reaction. These findings can be utilized for tuning the sensitivity of fluorescent probes for specific applications.

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© 2003 American Chemical Society

Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Fluorescent probes, molecules whose emission responds to changes in their direct environment, are frequently used as analytical tools for monitoring photoinitiated polymerization processes. Fluorescent probe technology is non destructive and often employed in real-time, and since fluorescence spectra can be recorded for samples of different sizes and geometries, fluorescent probe technology is suitable for on-line monitoring of photocured products. During a polymerization process, the anisotropy, the wavelength λ ^ χ , the intensity Imax, and the half width of emission of the addedfluorescentprobe may change. Our research is focused on charge transfer probes {1,2) whose wavelength λ , ^ strongly shifts to the blue due to increases in microviscosity as the medium polymerizes. These probes are very sensitive, and their application requires no calibration of individual samples. Monitoring a photoinitiated polymerization by real-time fluorescence, using the excitation light for simultaneous excitation of probe and photoinitiator molecules, is easily automated by recording an intensity ratio R= Ιλι/Ιλ instead of λ , ^ , and this procedure has been used for determining photoinitiator efficiencies. (3,4) 2

Ο Scheme 1. Fluorescent probes 1-7. Until recently, developing more sensitivefluorescentprobes usually came down to finding more sensitive chromophores. However, we have demonstrated that strong increases in probe sensitivity can be obtained by attaching reactive groups to the chromophore, i.e. by making reactivefluorescentprobes. (5,6) The



428 increase in sensitivity is very pronounced in network forming di(meth)acrylates and is clearly linked to the distribution of chromophores throughout a highly inhomogeneous polymerizing medium.(

Experimental HEXDMA (1,6-hexanediol dimethacrylate) and the starting materials for the synthesis of 1-7 were purchased from Aldrich and used as received. The fluorescent probes 1-7 were synthesized by esterification of the corresponding diol, as described earlier for 2 and 4. (5) The photoinitiator Irgacure 907 (2methyl-l-[4-(methylthio)phenyl]-2-moφholinopropanone-l) was a gift from Ciba-Geigy. Stock solutions containing 1.00% of photoinitiator in HEXDMA were prepared. To each of these solutions fluorescent probe was added to make 3.0* 10" mol kg" solutions. A few drops of these formulations were squeezed between microscope slides held apart by 75 μπι polypropylene spacers, with 50*15 mm windows, while clamps on both edges held the slides together. The surface of the top slide was made hydrophobic by treatment with hexamethyldisilazane, to ensure a reproducible detachment of the shrinking polymerfromthe top slide. (9) Samples were placed directly on the head of a C M 1000 cure monitor (3,4) and small spots («3 mm in diameter) on the layer were cured by the excitation beam. Plots of the intensity ratio R= Ι614/Ι654» versus the irradiation time t, were constructed by averaging 3-4 consecutive real-time measurements performed on the same sample. Double bond conversions were determined by IR measurements on 15μπι films between NaCl plates using a Mattson 6020 Galaxy Series FT-IR spectrometer. First derivatives of the intensity 4

1


429 ratio R versus the conversion C, (dR/dC), were determined numerically employing the software package Table curvefromJandel.

Results and Discussion


Comparing Reactive and Corresponding Non-reactive Probes. Real-time fluorescence performed on formulations containing added fluorescent probes results in plots of an intensity ratio R versus the irradiation time t. At low and equal [probe] the rate of the polymerization is identical for all formulations and therefore one can directly compare the overall sensitivities of all probes. The normalized intensity ratios R mr (I6i4/l654)r(l6i4/l654)t=0 (10) for 1-4 are depicted in Figure 1. QO

0

200

400

600

800

1000

Irradiation Time (s)

Figure 1. Normalized intensity ratio R HEXDMA, versus the irradiation time t.

norm

=

( W W M W I O S ^ O

for 1-4 in

From Figure 1, the overall sensitivities are determined: 2 > 1 » 3 > 4 . It is obvious that he reactive probes 1 and 2 are far more sensitive than the nonreactive probes 3 and 4. However, within each class differences in sensitivity are observed; 2 is significantly more sensitive than 1, whereas 3 is slightly more sensitive than 4.


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The curves in Figure 1 are a product of the rate of the polymerization, C versus t, times the responses of the individual probes, R versus C. As the rate of polymerization largely determines the shape of the curves in Figure 1, and since we are interested in the probe sensitivity primarily, we have plotted R as a function of C for 1-4 in Figure 2. For all probes the gradient of the curves in Figure 2 increase monotonically as the reaction proceeds, indicating that the probe sensitivity, defined as (dR/dC), increases at higher conversions. This effect is most pronounced for the non-reactive probes 3 and 4, primarily because they are relatively insensitive at low conversions.

0

0.2

0.4

0.6

0.8

1

Conversion

Figure 2. Normalized intensity ratio R HEXDMA, versus the conversion C.

norm

=

(I i4/l654)r(l6i4/Io54)t=o 6

of 1-4 in

In Figure 3, the sensitivity (dR/dC) for 1-4, obtained by numerical differentiation, is plotted as a function of the conversion C. From figure 3, the order of sensitivity of 1-4 at all stages of the reaction is determined. For all probes, the sensitivity increases strongly throughout the reaction; by a factor 3-5 for reactive probes, and by a factor 12-14 for non-reactive probes. Differences in sensitivity between the non-reactive probes 3 and 4 are very small, and 3 is slightly more sensitive than 4 at all stages of the reaction. For the reactive probes 1 and 2, differences in sensitivity are more pronounced. At the beginning of the reaction 2 is significantly more sensitive than 1. A reversal of sensitivity is observed at 59% conversion, and above that conversion 1 is more sensitive than 2. It should be noted that differences in sensitivity are particularly large at low conversions, with 2 being twice as sensitive as 3 and 4 at conversions below 10%.



431 From Figure 3 we can determine which probe is the most sensitive at all stages of the reaction. Up to a conversion of 59% 2 is the most sensitive probe. Above 59% conversion 1 takes over, up to a conversion of 87%, above which 3 is the most sensitive probe. It should be noted that at high conversions differences in sensitivity between 3 and 4 appear to be negligible, and that for HEXDMA conversions above 90% are not likely to be achieved. An alternative method for determining the relative sensitivities of 1-4, at all stages of the reaction is by plotting the difference of intensity ratios AR of two selected probes as a function of the conversion C, see Figure 4. From the gradient of these curves one can determine which probe is the most sensitive. Since this method does not require any data manipulation, apart from transforming the Xaxis from time to conversion, it is preferred for cases in which differences in probe sensitivity approach the limits of detection. From Figure 4, one can deduce that 2 is more sensitive than 1 up to a conversion of 62%, more sensitive than 4 up to a conversion of 78%, and that 3 is more sensitive than 4 at all stages of the reaction. Furthermore 1 is more sensitive than 3 throughout the entire reaction, but it appears that a reversal of sensitivity will occur around an 85% conversions. Clearly these observations are in agreement with those extractedfromFigure 3, indicating that identical results are obtained by both procedures. The relative sensitivities of 1-4 are explained in a straightforward manner by taking into account the reactivities and the sizes of the substituents attached to these probes. Comparing the sensitivity of the non-reactive probes 3 and 4, only size can play a role. (7) Probe 3, the smaller probe, is found to be slightly more sensitive throughout the reaction, indicating that decreasing the size of a probe increases its sensitivity. The larger sensitivity of 2 over 1 during the first 60% of the polymerization is explained by taking into account the reactivities, and the sizes of methacrylates and acrylates. Based on reactivity ratios (77) it is expected that methacrylates will be incorporated in the network at higher rates during the initial stages of the reaction. (72) This will result in a higher sensitivity of 2 at low conversions. Differences in size will play an opposite role, increasing the sensitivity of unreacted 1. Since 2 is the more sensitive probe at low conversions, we conclude that differences in reactivity have the largest effect on the sensitivity of both reactive probes. This consistent with the small differences in sensitivity between 3 and 4, the compounds that mimic the behavior of unreacted 1 and 2. When comparing reactive and corresponding non-reactive probes, 1 versus 3 and 2 versus 4, the largest differences in sensitivity are observed. Covalent attachment of reactive probes to the polymer network, increasing the rate of probe incorporation in rigid highly cross-linked areas, explains the large increase in sensitivity of the reactive probes over the non-reactive probes. The increase in sensitivity of 2 compared to 4 is much larger than the increase in sensitivity of 1


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5

Ο

0.2

0.4

0.6

0.8

1

Conversion

Figure 3. First derivative of the intensity ratio R versus the conversion C, (dR/dC), plotted against the conversion C. 0.4

Conversion

Figure 4. Differences in intensity ratio versus the conversion C.

A R = R be rRprobe 2 pro

fo* selected probes


433 over 3. This is explained by taking into account the effects of reactivity, favoring 2 over 1, and size, favoring 3 over 4, although the magnitude of the last effect is expected to be negligible. At very high conversions non-reactive probes are more sensitive than their non-reactive counterparts. This might be due to a higher concentration of non-reactive probes in the few remaining mobile monomer rich regions, resulting in a higher rate of probe incorporation in the network as these regionsfinallypolymerize.


Comparing Non-reactive Probes with Substituents of Different Sizes. (7) From the results presented in the previous section it was concluded that not only the reactivity, but also the size of the substituents effect the sensitivity of structurally similar probes. In order to investigate the effect of probe size in more detail we have employed 3 and 5-7. In this series 3, 5,6 the ester is linear and extended by methylene groups, and the weight of the probe increases while the polarity decreases. The pivaloyl ester 7 is the branched analogue of the valerate 6. The molecular weights of 6 and 7 are identical, but the mobility of 7 might be reduced due to steric hindrance. Plots of R versus t and C for 3 and 5-7 in HEXDMA are depicted in Figures 5 and 6. Obviously the differences in sensitivity between these non-reactive probes are small, compared to those between reactive and non-reactive probes. Based on the order of the overall sensitivity, 5>3>6>7, it is concluded that the sensitivity decreases as the size of the substituent increases. The effect of branching on probe sensitivity is very small, and it appears that branching decreases probe sensitivity since 6, is slightly more sensitive than 7. By subtracting selected curves the order of sensitivity throughout the polymerization is determined, see Figure 7. It is concluded that larger probes are less sensitive at all stages of the polymerization reaction. Figure 7 also shows that the decrease in sensitivity due to branching occurs at high conversions only. This brings us to the conclusion that the size of a probe influences the probe distribution during the photoinitiated polymerization. Larger probes are less sensitive, and therefore enriched in mobile monomer rich regions to a larger extent. This may be caused by a more effective exclusion of larger probe molecules from cross-linking regions, a process known as photodiffusion. In addition diffusion of probe and monomer molecules back into cross-linked regions, a swelling process, may occur and this process is expected to be faster and occurring to a larger extent for smaller molecules, due to steric constraints imposed by the network. It is expected that for both processes the relative size of the probe compared to the mesh size of the network will be a decisive factor.


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1.5

Ο

200

400

600

800

1000

Irradiation Time (s)

Figure 5. Normalized intensity ratio R nn HEXDMA, versus the irradiation time.

=

(IOI^WMIOIVIOS^O

n0

0

0.2

0.4

0.6

of 3 and 5-7 in

0.8

1

Conversion

Figure 6. Normalized intensity ratio Rnoim HEXDMA, versus the conversion C.

=

(l6i4/l654)r(Ï6i4/l654)t=o

of 3 and 5-7 in


435

0.08 i

5-6

0.05 H

5-3

0.02 j

6-7


Photoinitiated Polymerization - American Chemical Society

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