Irradiated polycarbonate films

The polycarbonate films exposed in this survey have received varying total irradiations. With the exception of films exposed to only the longest wavelengths (X > 337 nm), all the exposed films show obvious changes in their physical and spectral characteristics indicating moderate to extensive damage. The physical changes are discussed below and summarized in Table II. 

The second type of deviation was the anomalous type, in which the rate of thermal cis - trans isomerization of azobenzene in polycarbonate film prepared by UV irradiation was initially faster than those in solutions and gradually approached the latter. Pmk and Morawetz attributed the anomalously fast matru effect to the trapping of some cis-isomers in a strained conformation during UV-irradiation below Tg. On the other hand, Eisenbach attributed this anomaly to the restricted chain... 

The redistribution of free voliunes also influences the sub-glass transition temperatures Tp and T observed for photoisomerization reactions in polymer solids. T, Tp and T are frequency-dependent, and the response of any process to the transitions at these temperatures depends on the time scale. The time scale of photoprocesses may not be equal to those of DSC or dynamic mechanical methods, which are of the order of 10 to 1( Hz. However, for photodecoloration of the merocyanine form of spiro-bepzopyran in polycarbonate film under steady-state irradiation of 560 nm light after laser-single-pulse induced coloration, it was found that the Arrhenius plot of the apparent rate coefficient broke at T (150 °C), Tp (20 C), and T (—120 °Q of the matrix polycarbonate these temperatures are the ones determined by dynamic mechanical measurements. The excited state lifetime of the merocyanine form in polycarbonate was 1.8 ns . Hence, the decolorating isomerization during the lifetime proceeded only in a small fraction of the molecules surrounded by a sufficient amount of free volume. Thus, it is likely that the temperature dependence of the apparent rate coefficient reflecting the relative quantum yield is controlled by the frequency of redistribution of free volumes, which may be comparable with the frequency determined by dynamic mechanical measurements. 

For example, the rate of thermal ds - trans isomerization of azobenzene in polycarbonate film prepared by UV irradiation is faster than the rates in solutions, and it gradually apjHoaches the value in solution as shown in Fig 13. This anomalous fast process in the beginning of thermal badkward isomerization was attributed by E ik and Morawetz to trapping of some portion of cis isomer in a strained conformation. 

Pig. 13. Thennal backward cis -+ trans isomerization of azobenzene at room temperature in polycarbonate films UV irradiated in advance onto the film (O). UV irradiated during casting ( ), as wen as of azobenzene in ethylacetate ( ), ethanol (A), and dichloroethane ( ) UV irradiated in advance... 

The membrane is produced by using an irradiation and etching process on polycarbonate film. In theory, this fabrication procedure allows to obtain uniform pores, approximatively cylindrical, as it is illustrated in Pig.l ... 

Figure 9 Ultraviolet spectral absorption of 2 mil polymer films and spectral irradiance of sunlight. Abbreviations PVC, Polyvinyl chloride PE, polyethylene PS, polystyrene PC, polycarbonate AP, aromatic polyester PSF, polysulfone PET, poly(ethylene terephthalate) PAR, polyacrylate. Source From Ref. 16.

FIGURE 43.7 Examples of porous structures produced in thin polymeric films using various methods of irradiation and chemical treatment. (Reprinted from Apel, P., Radial. Meas., 34, 559, 2001. With permission from Elsevier.) (A) Cross section of a polycarbonate membrane with cylindrical nonparallel pore channels (B) polypropylene membrane with slightly conical parallel pores (C) polyethylene terephthalate membrane with cigar-like pores and (D) polyethylene terephthalate membrane with bow-tie pores. 

Finally, track-etched MF membranes are made from polymers, such as polycarbonate and polyester, wherein electrons are bombarded onto the polymeric surface. This bombardment results in sensitized tracks, where chemical bonds in the polymeric backbone are broken. Subsequently, the irradiated film is placed in an etching bath (such as a basic solution), in which the damaged polymer in the tracks is preferentially etched from the film, thereby forming cylindrical pores. The residence time in the irradiator determines pore density, and residence time in the etching bath determines pore size. Membranes made by this process generally have cylindrical pores with very narrow pore-size distribution, albeit with low overall porosity. Furthermore, there always is the risk of a double hit, i.e., the etched pore becomes wider and could result in particulate penetration. Such filter membranes are often used in the electronic industry to filter high-purity water. 

Swift heavy ion beams with extremely high LETs are used to degrade strongly polymer chains within tracks over a range of a few tens of micrometers. After irradiation, the top surface of bulk thermoplastics such as polyethyleneterephtalate, polycarbonate, polyvinylidene fluoride or polyimide materials as well as thin films made of the same material are subsequently etched by a wet chemical treatment that reveals pores of which the shape, surface density and dimensions can be controlled by choosing appropriate conditions. High aspect ratio cylindrical or conical traces of diameter ranging from a few nanometers to some... 

Acid Catalyzed Hydrolysis. When photosensitive onium salts are incorporated into a film of a hydrolytically sensitive polycarbonate and exposed to ultraviolet irradiation, the acid which is produced catalyzes chain scission at random sites along the backbone (Equation 25). 

When polycarbonate is irradiated in the solid state, yellowing is confined almost exclusively to the surface, since the light intensity penetrating the sample decreases as the dose is increased. Change in carbonyl absorption and increase in hydroxyl contents have been detected together with formation of phenols, esters, ethers and acids, and evolution of carbon monoxide and dioxide [110]. Since crosslinking is associated with chain scission, a thin yellow film coming from the surface remains insoluble after dissolution of the irradiated sample in methylene chloride... 

Classification by Decomposition Behavior. The decomposition mechanism is a reasonable way to classify polymers. They can either depolymerize upon irradiation, for example, poly(methylmethacrylate), or decompose into fragments such as poly-imides or polycarbonates. This method of classification is closely related to the synthesis of polymers. Polymers that are produced by radical polymerization from monomers, which contain double bonds, are likely to depolymerize into monomers, while polymers that have been formed by reactions like polycondensation will not depolymerize into monomers upon irradiation, but will be decomposed into different fragments. The second group cannot be used to produce films with the same structure or molecular weight as the original material with methods such as PLD. 

In addition to the above, photo-Fries rearrangements occur in various aromatic polyester films upon ultraviolet light irradiation [560, 561]. Also, photo-Fries rearrangements are believed to occur in polycarbonate resins as well [562]. This can be illustrated as follows ... 

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