Photoinitiated Polymerization of Liquid Crystalline ... - ACS Publications

Chapter 9

Photoinitiated Polymerization of Liquid Crystalline Monomers Bearing Nonaromatic Mesogenic Units Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch009

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C. E. Hoyle , A. C. Griffin , D. Kang , and C. P. Chawla 1

Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406 Melville Laboratory for Polymer Synthesis, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom

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There have been a number of papers appearing in the literature over the past two or three decades which have focused on the effect of conducting polymerization in liquid crystalline and organized media. Two excellent reviews of the subject describe the state of the literature of the early 1980s (1, 2). The basic conclusions of the two review articles indicated that, although there was some evidence to support the supposition that polymerization of a liquid crystalline monomer or polymerization of an isotropic monomer in an inert liquid crystalline solvent could result in enhanced polymerization rates, no definitive investigations dealing with the consequences of polymerization of mesomorphic systems had been conducted. This was the case until the classic work by Broer and coworkers (3-6) in the late 1980's. The work of Broer leaves little question that polymerization of liquid crystalline monomers can result in quite different kinetics for chain formation. In addition, Broer provided results which indicated that orientation induced in monomeric liquid crystalline films can be retained in the polymers subsequently formed. Furthermore, in a recent report, Broer, Mol, and Challa (7) show that chain transfer processes which normally occur in diacrylates are minimized upon polymerization in organized media. All of the compounds investigated by Broer have groups comprised of arylbenzoate mesogenic units separated by alkyl spacer groups from the reactive acrylate moiety. In this paper we report results for the kinetics of the photoinitiated polymerization of two liquid crystalline monomers which have non-aromatic mesogens. The first monomer has a cholesteryl mesogen separated by a flexible alkyl spacer from a reactive methacrylate. The second is a semifluorinated acrylate monomer.

0097-6156/93/0527-0118$06.00/0 © 1993 American Chemical Society

In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch009

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Photoinitiated Polymerization of Monomers

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Photoinitiated polymerization of the cholesteryl methacrylate monomer has been described by Shannon in a series of articles (8, 9) and patents (10,11) which, along with the work of Broer, must be classified as truly exceptional collections describing the virtues and opportunities for polymerization of mesogenic monomers. In an initial paper, we have reported on some of the kinetic aspects of a cholesteryl methacrylate monomer with a long alkyl chain spacer group (12). In this paper, we report on the use of a pulsed laser to provide additional information. By using a laser fired at quite wide intervals, it is possible to clearly identify the rate acceleration which accompanies changes in the polymerization medium. In the case of the functionalized semifluorinated acrylate monomer, we find an unusually rapid polymerization rate which, if optimized, can give conversion efficiencies on the order of 70-80 percent from a single laser pulse. Experimental The synthesis of CMA-10 followed a literature procedure (8,9). F12H10A was synthesized by a method similar to that reported in reference 13. The laser employed was an excimer laser from Questek or Lumonics. Single shot powerper-pulse values were obtained from power measurements recorded at higher repetition rates and are therefore subject to some error. The DSC was either a modified Perkin Elmer IB or Perkin Elmer 2B. In all cases except for the data in Figure 5, samples were cast in indented DSC pans. For the results in Figure 5, samples were cast in DSC pans after a simple washing procedure. Results in Figure 5 are thus only qualitative since the sample was not uniformly distributed across the pan but agglomerated near the edges in a ring, α,αDimethoxyacetophenone (DMAP) was obtained from Ciba-Geigy. For quantum yield measurements a filtered (366 nm) medium pressure mercury lamp was employed with a nominal output of 2.16 χ 10' mJ cm' sec' at the sample pan. The heat of reaction of CMA-10 was taken as 13.7 kcal mole" . The heat of reaction of F12H10A was estimated as 22.5 kcal mole" by assuming 99% conversion after 30 laser pulses and utilizing the resultant area (in kcal) under the exotherm curve. (We note that absolute quantum yield values may be subject to considerable error). 2

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Results and Discussion In order to illustrate the types of behaviour which can be observed during the photoinitiated polymerization of a liquid crystalline monomer to give a liquid crystalline polymer, two monomers which have liquid crystalline phases have been evaluated. The first is a methacrylate monomer with a pendant cholesteryl mesogenic group decoupled from the methacrylate functionality, and subsequently the polymer generated, by a sequence of ten methylene spacers. Some of the properties of this monomer have been presented in the past (8, 9). The cholesteric monomer presents an opportunity to investigate the polymerization kinetics in liquid crystalline media characterized by mobility that is somewhat limited. The second monomer investigated is a semifluorinated acrylate which exists in a highly ordered smectic Β type phase. The kinetics for the semifluorinated methacrylate results are reflective of greater restrictions on the polymer kinetic chain mobility. In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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IRRADIATION OF POLYMERIC MATERIALS


Cholesteryl Monomer. The cholesteric methacrylate monomer, which we designate as CMA-10, exists in a smectic phase below 52 °C and a cholesteric

phase from 53 °C to 64 °C as determined by differential scanning calorimetry and polarized optical microscopy. (Transition temperatures are slightly lower when low concentrations of photoinitiator are added to the pure monomer). In the first set of experiments designed to elucidate the polymerization profiles of CMA-10 as a function of the percent conversion, single pulses from an excimer laser (XeF=351 nm) were focused onto a DSC pan containing CMA10 with 1 wt percent α,α-dimethoxyacetophenone (DMAP). Figure 1 shows results for the cumulative percent conversion versus time plots at 45 °C obtained from intergration of the individual exotherms generated by laser pulses of approximately 10 nanoseconds duration and spaced 15 seconds apart. The percent conversion was calculated using a value of 13.7 kcal mole' as the enthalpy of polymerization of CMA-10. The most notable feature of the percent conversion versus time plot in Figure 1 is the continuous and smooth buildup in the primary curve followed by a limiting percent conversion of 88 percent. We have reported that during polymerization at 45 °C there is little or no change in the smectic texture (see references 8 and 9 for original reports on texture at this temperature) of the medium as viewed by cross-polarized optical microscopy, even at higher percentages of conversion. The absence of any significant abrupt change in the plot in Figure 1 is consistent with the observation that the medium texture does not change appreciably during the photoinitiated polymerization process. When the polymerization of CMA-10 at 58 °C (Figure 2) is initiated with laser pulses spaced 15 seconds apart, at very short times the conversion is significantly lower than for subsequent pulses. This is a reasonable result if one keeps in mind that the polymerization of CMA-10 results in an abrupt change to a biréfringent medium with different texture from the monomer at 58 °C (cholesteric) as identified by cross-polarized at conversions of about 1-2 percent. The birefringence after the medium change has been postulated to arise from a smectic structure. 1


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Photoinitiated Polymerization ofMonomers

CMA-10


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Figure 1.

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Cumulative conversion plot for CMA-10 film (1 wt percent DMAP) at 45 °C from exotherms initiated by XeF (351 nm) laser pulses (0.64 mJ cm" pulse' ; 15 seconds apart). 2

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Figure 2.

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Cumulative conversion plot for CMA-10 film (1 wt percent DMAP) at 58 °C from exotherms initiated by XeF (351 nm) laser pulses (0.64 mJ cm" pulse" ; 15 seconds apart). 2

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If the laser-initiated polymerization is conducted with the CMA-10 monomer initially in the pure isotropic phase, i.e, at 75 °C and 88 °C., two observations are made from an examination of the percent conversion versus time plots in Figures 3 and 4. First, in each case the percent conversion for the first few laser pulses is markedly lower than for subsequent pulses. This is an equivalent way of stating that there is a discontinuous break in the percent conversion versus time plot for polymerizations conducted at temperatures in which the monomer is initially in the isotropic or cholesteric phase. As in the case of polymerization at 58 °C where the monomer is initially in the cholesteric phase, the medium converts to a low order liquid crystalline medium with grainy texture similar to that observed at 58 °C during the course of the polymerization process. This no doubt correlates with the increase in the polymerization efficiency after the first few laser pulses. If we project that the medium change begins at the intersection of tangent lines drawn through the percent conversion versus time plots before and after the increase in polymerization rate, it is apparent that the percent conversion required to obtain the texture change and accompanying rate increase is higher at greater temperatures. In order to provide additional characterization for the polymerization of CMA-10 at 75 °C and 88 °C., significantly reduced (in intensity) pulses from the laser (k = 351 nm) were spaced 200 milliseconds apart, i.e., the laser was operated at a repetition rate of 5 Hz. The exotherm curves were then recorded for delivery of selected numbers of total pulses to the CMA-10 sample in the DSC. Figure 5 shows results for CMA-10 at 75 °C and 88 °C. If only 10 pulses are delivered to each CMA-10 sample at 75 °C and 88 °C., the exotherm curve at 88 °C is larger and has accordingly a higher area (extent of conversion) than the exotherm curve for the sample at 75 °C subject to 10 pulses. If 20 pulses are delivered to each sample, the exotherm areas are approximately identical, even though the peak maximum of the exotherm curve for CMA-10 at 88 °C is still somewhat higher. The results obtained for 10 pulses reflect polymerization of the monomer strictly in the isotropic phase of the monomer prior to any (or at least very little) change in the reaction medium: at both 75 °C and 88 °C the percent conversion is well below 3 percent. We conclude that polymerization in the isotropic phase is faster at 88 °C than 75 °C. However, for 20 pulses, the percent conversion is greater than for 10 pulses and the areas under the exotherms are approximately identical. This reflects the fact that part of the polymerization at 75 °C occurs after the abrupt change in the medium, and the polymerization rate is accelerated. In the case where a larger number of pulses (25 or 35) are injected into the sample, the conversion is higher at 88 °C than at 75 °C. At 75 °C a significant part of the polymerization actually takes place in the liquid crystalline medium since the total percent conversion is well above 5 percent in both cases and the change from an isotropic to a biréfringent liquid crystalline medium occurs at about 2 percent conversion. Thus, the plots in Figure 5 illustrate, as do the result in Figures 3 and 4, that polymerization after the medium change is more efficient than in the isotropic phase: even if the sample system is at a lower tx


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Photoinitiated Polymerization of Monomers

CMA-10 AT

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temperature. This is interesting since the natural effect of temperature, as illustrated by the results in Figure 5 for only 10 pulses, is to increase the rate. To provide additional, and somewhat more quantitative, information about the efficiency of the polymerization of CMA-10, we have calculated polymerization quantum yields for CMA-10 at 88 °C. Figure 6 provides a pictorial of our procedure for obtaining quantum yields. Initally, each sample is exposed to the output of a medium prerssure mercury lamp (filtered with a 366 nm band pass filter) via a rapid shutter until a given percent conversion is attained: the percent conversion is calculated from the integrated area under the exotherm curve. After allowing the exotherm to return to a baseline value, the sample is exposed by shuttering the mercury lamp for a short period of time. This generates a small exotherm from whence the percent, and thus molar quantity, of monomer converted by the second shuttered exposure can be calculated. By knowing the exact intensity of the lamp at 366 nm using a black body absorber in the DSC pan, the quantum yield of polymerization can be calculated. Approximate lower limit quantum yields for CMA-10 at 88 °C obtained via the method in Figure 6 are lower in the isotropic phase (Φροί, *ο " 3700) than in the medium (

Photoinitiated Polymerization of Liquid Crystalline ... - ACS Publications

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