PVCa

Table 3.21 Extraction of plasticiser (DOP, DINP) from flexible PVCa i>...

In the case of undoped PVCa films, impurities and surface states dominate the photoconduction mechanism (6) leading one to question any study of intrinsic pKotoconduction in organic polymers. Poly(N-vinylcarbazole) films yellow under ambient laboratory conditions. Work in our laboratory (7) has shown that ageing of a purified sample of PVCa leads to an increase in photoresponse in the 350-450 nm region while there is an initial drop in photoresponse in the 250-... 

The photodegradation of PVCa solutions has been studied to determine a mechanism for the photodegradation process and identify the excited states involved. The polymer was secondary grade (Aldrich Chemical Company) that was further purified by reprecipitation three times from methylene chloride solution by the dropwise addition into methly alcohol. Solutions in methylene chloride were found to yellow rapidly in air upon exposure to 360 nm light, whereas solutions degassed by the freeze-pump-thaw technique showed little yellowing. 

Figure 3. Stern-Volmer plots for quenching of yellowing following photolysis of PVCa solutions in methylene chloride by (a) piperylene and (b) naphthalene. Yellowing is measured as the increase in absorption at 390 nm.

Films of purified PVCa were cast from methylene chloride solution on quartz plates. The solvent was allowed to slowly evaporate to give smooth, clear films with a thickness of ca. 5.0 nm. Contact angle measurements using water droplets were measured with a standard contact angle goniometer. Samples were photo-lysed in air with polychromatic light from a 150 watt xenon arc. Contact angles were measured after various times of irradiation to monitor the formation of oxidation products at the surface of the polymer films. 

Results are shown in Figure 5. Samples of PVCa were doped with 10% by weight of poly(1-vinylnaphthalene) to determine if the naphthalene chromophore would serve as a quencher for the surface oxidation of PVCa as it appears to do in the case of fluid solutions. 

The data in Figure 5 indicates that the presence of the naphthalene chromophore does not prevent the decrease in contact angle observed upon the irradiation of PVCa. Thus, naphathalene may rot be an effective quencher of the surface oxidation of PVCa film as it appears to be for the yellowing of PVCa in solution. 

The surface oxidation products dete ted by the decrease in contact angle upon photolysis of PVCa films may dominate the photoconductivity of t. is polymer. Work is underway to confirm this relatio. ship and measure surface conductivity simultaneously with bulk conductivity as a function of photodegradation. 

Work is continuing to correlate the formation of surface oxidation states with changes in the photoconduction of films of PVCa. The relationship between energy transfer and photoconductivity is being investigated. 

Polyethylene Polypropylene PVCa ABS PTFE Acrylics Nylon 66 Acetal... 

PS PSF PSU PTFE PU PUR PVA PVAL PVB PVC PVCA PVDA PVDC PVDF PVF PVOH SAN SB SBC SBR SMA SMC TA TDI TEFE TPA UF ULDPE UP UR VLDPE ZNC Polystyrene Polysulfone (also PSU) Polysulfone (also PSF) Polytetrafluoroethylene Polyurethane Polyurethane Poly(vinyl acetate) Poly(vinyl alcohol) poly(vinyl butyrate) Poly(vinyl chloride) Poly(vinyl chloride-acetate) Poly(vinylidene acetate) Poly(vinylidene chloride) Poly(vinylidene fluoride) Poly(vinyl fluoride) Poly(vinyl alcohol) Styrene-acrylonitrile copolymer Styrene-butadiene copolymer Styrene block copolymer Styrene butadiene rubber Styrene-maleic anhydride (also SMC) Styrene-maleic anhydride (also SMA) Terephthalic acid (also TPA) Toluene diisocyanate Ethylene-tetrafluoroethylene copolymer Terephthalic acid (also TA) Urea formaldehyde Ultralow-density polyethylene Unsaturated polyester resin Urethane Very low-density polyethylene Ziegler-Natta catalyst... 

Table III. Fractional Precipitation of Graft Copolymer of 50% EVA (45% VAc) and 50% PVCa...

PVCA is a copolymer of vinyl chloride and vinyl acetate. It is a colorless thermoplastic solid with good resistance to water as well as concentrated acids and alkalis. It is obtainable in the form of granules, solutions, and emulsions. Compounded with plasticizers, it yields a flexible material superior to rubber in aging properties. It is widely used for cable and wire coverings, in chemical plants, and in protective garments. 

Neat Polymer Films. It is interesting to compare the triplet photophysical properties of poly(N-vinylcarbazole) (PVCA)(16) on the one hand and poly(l-vinylnaphthalene) (PIVN)(17) on the other when each is examined as a pure polymer film. Both polymers exhibit a prominent excimer phosphorescence band as well as a distinct delayed fluorescence emission. In addition, the delayed fluorescence arises by a process of triplet-triplet annihilation for both polymers. Furthermore, the luminescence decay kinetics suggest that equilibria of the type... 

An important difference in photophysical behavior between the naphthalenic and carbazole polymers is the lifetime of the mobile exciton compared with that of the triplet excimer. For PVCA the mobile exciton is much shorter lived than the excimeric species but for PIVN just the reverse is true. In both polymers the primary mode of delayed fluorescence production involves a hetero-annihilation of the type... 

Thus, the delayed fluorescence lifetime is essentially equal to that of T for PVCA and to T for PIVN. The latter is easy to prove since excimer phosphorescence nd delayed fluorescence lifetimes are the same for PIVN. For PVCA at 77 Kt x. but no direct and independent measure of the lifetime of T Tn PyCA has been accomplished at this time. 

Furthermore, the diffusion coefficients for triplet exciton migration extracted from this three-dimensional intramolecular model were nearly the same as those obtained using the conventional kinetic equation (i.e. eq 1). The hopping frequencies for triplet exciton migration in PVCA for these three models are summarized in Table 111(20). Neither the electron exchange mechanism(21) nor the Forster... 

Table III. Frequencies of Triplet Exciton Migration for Rigid Solutions of PVCA in MTHF at 77 K Using Various Models for...

Time-Dependent Phosphorescence Spectra of Polymers. Although a strong similarity exists between phosphorescence spectra of vinyl aromatic polymers and the corresponding monomeric analogues, it is interesting to focus attention upon the differences which exist between these spectra. A sample comparison is provided in Figure 1 between NEC and PVCA both as dilute solutions in MTHF at 77 K (23). Both spectra are recorded using comparable conditions and instrument parameters. 

In the spectrum of PVCA it seems that the 0-0 emission band is present only as an unresolved shoulder and that the remaining structural features of this spectrum are somewhat broadened compared with those of NEC. An attempt was made to achieve better resolution of the 0-0 band of PVCA using narrow monochromator slits and multiple scans of the emission band. In addition, spectra were recorded using variable delay times following excitation. These results are presented in Figure 2. 

Two observations about the 0-0 phosphorescence band of PVCA emerge from these experiments. In the first place a clear resolution of this band is, in fact, achieved. In addition, the intensity of this band decreases relative to longer wavelength components by using long delay times on the order of several hundred milliseconds. An obvious corollary to this effect is that apparent phosphorescence decay times will depend upon the wavelength chosen for the measurement. Such effects are not large but they are readily measurable. 

In order to demonstrate this effect to best advantage it was necessary to choose a PVCA sample having a relatively low molecular weight. In this way Interference of the phosphorescence emission by delayed fluorescence is minimized. These are provacative results because they indicate that there may be no well defined lowest triplet state in vinyl aromatic polymers unless special steric or electronic effects are present which nullify inter-chromophore interactions. On the other hand, they may provide an additional tool with which to investigate rates of energy migration in polymers and in some polymer/dopant systems as well. 

Figure 1. Phosphorescence spectra of NEC (upper) and PVCA (lower) in solution at 77 K, Both spectra are recorded at 50 msec following the excitation pulse using a monochromator band pass of 6 nm.

Figure 2. Time resolved phosphorescence spectra of PVCA in solution at 77 K using a monochromator band pass of 2 nm. Delay times of 400 msec (upper) and 800 msec (lower) were used.

There is no significant difference between radically and cat-ionically prepared PVCA samples although the tacticity is reported to be different (12) and other spectral properties differ drastically (13-15). The results obtained on PVCA confirm earlier work by Okamoto et al. (16) reporting hypochromy data on several aromatic polymers in liquid solution obtained by graphical band separation. 

UV-absorption studies on aromatic polymers at low temperature solid glasses seem to be scarce. In Figure 1, the low temperature spectra of PVCA and its monomeric model N-Methyl-carbazole in MTHF glass at 77 K are compared (17). The monomer clearly shows better resolution due to sharper absorption bands in the two lowest transitions but again the hypochromic effect in the polymer is evident. 

Figure 2 shows typical solid solution spectra of cationically prepared PVCA and 1,3-Biscarbazolylpropane (BCP). The main difference of thia polymer spectrum relative to the low molar mass model is a small red shift which is also observed in other aromatic polymers (Table I), with exception of the more open-structured Poly(2-naphthylmethylacrylate). In the case of P2VN no monomer data for exactly the same solvent and temperature could be found in literature, the blue shift indicated may therefore be fortuitous. 

Figure 1. UV-absorption spectra of PVCA and monomeric model N-methylcarbazole replotted after Schneider (17)...

Figure 2. Fluorescence spectra of PVCA cat and PCP. The emission intensities of polymer and dimeric model are not comparable the excitation spectrum is not corrected for equal quantum intensity excitation at 330 4 nm...

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