Acetone Absorption in UV-Irradiated Polycarbonate.
Photodegradation of polycarbonate (PC) has been studied extensively for several decades, documenting scission and cross-linking reactions, changes in mechanical properties, and the evolution of degradation products [1-4]. It is well established that photodegradation occurs via two wavelength-dependent mechanisms, photo-Fries rearrangement and photo-oxidation. In 1966, Bellus et al. identified Photo-Fries reactions in a chloroform solution of PC irradiated with 254 nm UV light produced via a medium-pressure mercury lamp . Later (1971-1973), Humphrey et al. conducted microsecond flash photolysis experiments on PC and model compounds in dichloroethane solution [6, 7], Results were interpreted to be due to a Photo-Fries rearrangement occurring via an intermolecular route, resulting in free radicals arising from energy transfer between the phenolate [pi]* state and the carbonyl group. Additional scission was thought to occur by hydrogen abstraction in the backbone. During this time, it became apparent that when samples are exposed to radiation at wavelengths similar to that reaching the earth from the sun (radiation containing UV components and blue-light regions), different degradation mechanisms proceed simultaneously [8, 9]. In 1992, Andrady et al. identified two regions in the ultraviolet range that induce changes in the optical absorption in PC , They found that wavelengths <300 nm and between 310-350 nm exhibit unlike quantum efficiencies that differ in the amount of yellowing produced by increasing amounts of radiation. At shorter wavelengths, PC underwent degradation by the photo-Fries pathway resulting in more yellowing and greater increases in UV absorption. At solar, visible wavelengths, degradation proceeded via oxidation mechanisms that form a series of other yellow products.
Researchers continued to refine experiments to develop accelerated testing that reliably predicts mechanisms and quantifies the effect of radiation intensity on degradation products. In 2009, Diepens and Gijsman  showed that when the spectral wavelength distribution was kept constant for different irradiation intensities, mechanisms remained unchanged. UV and IR and fluorescence data demonstrated that there is a liner relation between intensity and photo-degradation product concentrations, proving that accelerated testing is a reliable method for predicting radiation damage. Soon after this, in 2011, Pickett demonstrated that PC photo-degradation is an auto-accelerating process wherein photo-Fries products initiate photo-oxidation of PC .
Importantly, positron tests conducted on PC were used to track free volume changes induced by exposure to UV radiation at 250 nm . Researchers found that Photo-Fries rearrangements resulted in a decrease in number density fractional free volume moieties. They concluded that "The formation of more stable free radicals leads to the production of a small number of large free volume holes." The carbonate linkages were shown to undergo radiation induced reactions, while phenyl rings were stable. It is interesting that E-beam irradiation has been shown to induce changes in free volume in PC that "erase" aging effects and significantly alter mechanical properties . Our interest in characterizing radiation effects on polymer matrices overlaps with our interest in probing events that induce volume changes in polymer matrices and on developing methods to detect these changes. Herein we probe UV-irradiated PC matrices via solvent sorption measurements.
Transport of organic solvent in polymers has been extensively studied for many decades. Both concentration-gradient-controlled-diffusion (Case I) and stress-relaxation-controlled-transport (Case II) influence the rate and amount of solvent uptake in polymers. In addition to the above two transport mechanisms, Alfrey et al.  proposed the third type of "anomalous" transport, which is encountered when both solvent mobility and stress relaxation rates are comparable. Hopfenberg and Frisch  noted that the type of penetration observed depends on temperature and penetrant activity. Kwei et al. [16-19] were the first to develop a mathematical model for anomalous transport, where penetrant boundaries advance at rates in between [t.sup.1/2] observed for Case I transport, and rates proportional to t observed in Case II transport.
Their equation was modified by Harmon et al. [20, 21] who studied methanol sorption in deformed poly(methyl methacrylate) (PMMA) with thin plate geometries. The Harmon model has been applied to many solvent-polymer systems [22-25]. For example, contributions from Case I and Case II transport have been quantified in poly(methyl methacrylate) (PMMA) [20, 21] and hydroxyethyl methacrylate (HEMA) copolymers . When solvents penetrate PMMA and HEMA, the polymers remain in the amorphous state. By contrast, during solvent sorption in PC, the microstructure changes from the amorphous to the crystalline state [22, 23, 26, 27]. Interestingly, solvent is initially absorbed in PC by Case I transport, but "squeezed out" by Case II transport.
Ware et al.  studied mass transport of methanol, acetone, and carbon tetrachloride in PC. The authors found that methanol sorbed via pure Case I transport and attributed this to be due to the fact that methanol is a somewhat poor solvent. Hence, the matrix is not plasticized enough to undergo solvent-induced crystallization. However, acetone and carbon tetrachloride, much better solvents for PC, exhibited anomalous transport wherein swelling relaxations influenced the kinetics. In these more highly plasticized systems, spherulites were observed. Miller et al.  reported Case I transport of carbon tetrachloride in PC at 25[degrees]C, but they neglected short sorption times. Diffusion fronts, at longer times exhibited square root of time dependency [22, 29, 30]. Furthermore, Wu et al.  studied acetone transport in gamma-irradiated PC, finding that the mechanism of acetone transport in the specimens at low temperatures is different from that observed at higher temperatures. The reason behind the change in transport mechanisms was not clear at that time. This prompted us to investigate acetone transport in UV-irradiated PC. Again, we demonstrate herein that the kinetics of mass transport measured over temperatures from -23[degrees]C to 20[degrees]C reveal that there is a critical temperature, [T.sub.c], at which the sorption mechanism changes. We conclude that the transport transition temperature of acetone in PC arises from changes in the crystalline lattice. Specifically, orthorhombic crystals are seen above [T.sub.c], whereas a mixture of orthorhombic and monoclinic crystals is observed below [T.sub.c]. This phenomenon is discussed in relation to solvent-induced crystallization, which varies with temperature and radiation dose. A comparison between the effects of UV irradiation and gamma irradiation on acetone transport in PC are presented in the discussion.
Lexan 8010 PC sheets were obtained from General Electric Company (Artesia, CA). Rectangular samples of dimensions 15 x 15 x 1 mm were used for mass transport studies and diffusion front measurements. The samples were ground with 1200 and 4000 grit emery papers, and then polished with 1 and 0.05 ixm alumina slurries. They were annealed at 130[degrees]C in air for 24 h, and furnace cooled to 25[degrees]C. The purpose of annealing was to reduce the residual stresses in samples arising from machining. Samples were then positioned in a chamber in air at 55[degrees]C and 85% relative humidity and were exposed to UV irradiation from a highly uniform 254 nm UV light (Giant Force Instrument Enterprise Co., New Taipei, Taiwan). The distance from sample to UV source was 1.5 cm. The intensity was measured by a 1918-C optical power meter at 0.35 mW/[cm.sup.2]. The samples were exposed to the UV source for 72, 120, and 168 h and the measured doses were 90.7, 151.2, and 211.7 J/[cm.sup.2], respectively.
Initial weights were recorded and the samples were conditioned at each absorption temperature and immersed in the acetone-filled glass bottles each temperature. A refrigerated circulating bath was used for temperature control. Weight gain was measured periodically via means of an OHAUS-AP250D digital balance (Pine Brook, NJ) and samples were returned to the acetone-filled glass bottles immediately. Periodic measurements were taken until samples reached a constant weight. Samples used for boundary measurements were acclimated similarly. Periodically, samples were removed from the bottle, immersed in liquid nitrogen for 1 min and cleaved. An Olympus-BHT optical microscope (Shinjuku-ku, Tokyo, Japan) was used to measure the penetration distance of the sharp, solvent front. The cleavage surface was also observed by an optical microscope.
Glass transition temperature ([T.sub.g]) studies were conducted on polished, annealed samples. Samples of approximately 10 mg were cut with a 5-mm-diameter punch. The samples were exposed to a UV source for different periods to reach the expected dose. Each sample was enclosed in a regular aluminum pan and placed in a Netzsch DSC 200F3 differential scanning calorimeter (Selb, Germany) for measurement. Samples were heated to 200[degrees]C and isothermed for 5 min to assure that any crystals melted. Samples were then quenched from 200[degrees]C to -50[degrees]C at a cooling rate of 40[degrees]C/min, and isothermed for 10 min. Finally, they were reheated at the rate of 10[degrees]C/min and these traces were used to determine glass transition temperatures. Molecular weights were determined via a Jasco Gel Permeation chromatography (GPC) (Jasco Product Inc., Essex, UK). GPC samples (about 3 mg) were dissolved in tetrahydrofuran (THF). JASCO 880-PC pump and JASCO 870-UV UV detector were used to measure retention times. The molecular weight of PC was calculated using a calibrated curved determined with polystyrene standards. Fourier-transform infrared (FTIR) spectroscopy was also conducted to characterize chemical changes in UV-irradiated PC. FTIR measurements were conducted on a Nicolet AVATAR 320 FTIR spectrometer in the attenuated total reflection (ATR) mode (Thermo Electron Corp., Madison, WI). Chemical changes were characterized via unnormalized peak heights. For the transport transition temperature study, the samples of 30 x 30 x 1 mm were used and reduced to thicknesses of 0.2 mm via polishing. After polishing and annealing, samples with 20% acetone obtained at temperatures of 30[degrees]C, 10[degrees]C, -2[degrees]C, - 10[degrees]C, and -20[degrees]C were moved to the FTIR sample compartment for the measurement. The resolution and scan times of FTIR were set at 4 [cm.sup.-1] and 128 times, respectively. The wavenumbers were in the range of 3800~850 [cm.sup.-1]. Density measurements were performed using an Ohaus density determination kit P/N 77402-00 (Cambridge, UK). The test liquid was the deionized water of density 0.99565 g/[cm.sup.3] at 30[degrees]C.
The crystalline structures of PC specimen was measured using SHIMADZU XRD 6000 X-ray Diffractometer (Shimadzu Corporation, Chiyoda-ku, Tokyo, Japan) with a copper X-ray source of wavelength 1.54056 [Angstrom]. The 2-theta angle ranged from 5[degrees] to 35[degrees] and the scanning rate was 2[degrees]/min. The size of PC specimens for XRD measurement was 8 X 8 X 1 mm. The specimens were immersed in acetone at 30[degrees]C and - 10[degrees]C. respectively, until saturated, and then diffraction patterns were recorded.
To examine the effect of additives on the absorption of PC, [sup.1]H MNR spectra of as-received and purified PCs were obtained by a Bruker AVANCE-500 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) using CD[Cl.sub.3] solvent. As received PC of 1 g added in dichloromethane of 20 ml was placed on the magnetic stirrer for 1 h to form a unified solution. Then acetone was added in dichloromethane solution for 10 min, the PC precipitate was collected by suction filtration through a PTFE membrane. The PC precipitate was dried in a chemical hood until all dichloromethane evaporated away. Both as-received PC and PC precipitate added in dichloromethane was placed on the magnetic stirrer of 300 rpm.
RESULTS AND DISCUSSION
GPC and DSC Studies
Table 1 lists the molecular weight of PC UV-irradiated with different dosses. Weight average ([M.sub.w]) weights decreased from 62,000 to 55,000 with increasing UV dose, while number average molecular weights ([M.sub.n]) decreased from 34,000 to 24,000. Glass transition temperatures of UV-irradiated PC with and without acetone absorption are tabulated in Table 2. It is found that glass transition temperatures decreased with increasing UV dose from about 151[degrees]C for the unirradiated sample to 146[degrees]C. This indicates that UV irradiation induced scissions in the PC chains. The results are consistent with literature data on glass transition temperatures and molecular weights for PC exposed to UV irradiation [3, 31-34], Lemaire et al. [3, 34] found that when PC is irradiated with a UV source with wavelengths <300 nm, Photo-Fries rearrangements take place. However, at wavelengths >340 nm, photo-oxidation plays an important role in degradation. The study herein used a UV wavelength of 254 nm; it is likely that photo-Fries rearrangements dominate the degradation process. As mentioned above, these rearrangements are thought to accompany scission reactions that are due to hydrogen abstraction [6, 7]
Wu et al.  studied the effect of gamma radiation on PC molecular weight using intrinsic viscosity measurements and found viscosity average molecular weights decreased linearly from 73,000 to 48,000 when the gamma-ray dose increased from 0 to 1000 kGy. The glass transition temperature also decreased from 147.5[degrees]C to 134[degrees]C at the same radiation doses. This indicates that the effect of gamma irradiation on glass transition temperatures and molecular weights of PC is somewhat more pronounced than effects noted with UV irradiation over the dosage ranges examined.
Photo-Fries Reaction and Crystallinity
Figure 1 shows the IR data for PC UV irradiated at different doses. After UV irradiation, two new absorption bands appear at 1689 and 1629 [cm.sup.-1]. These bands correspond to phenylsalicylate and 2,2'-dihydroxybenzophenone, respectively, as identified by Rivaton [2, 3]. Both peak intensities at wave-numbers 1689 and 1629 [cm.sup.-1] increased with increasing UV dose. These results are indicative of photo-Fries products in the UV-irradiated PC. Photo-Fries reactions consist of carbonate units rearranging to form phenyl-salicylate and 2,2'-dihydroxybenzophenone derivatives . In PC, the C--O bonds are weak, so the oxygen atoms rearrange to form to stronger O-H bonds, producing phenyl-salicylate structures , Then, the C-Os in phenyl-salicylate molecules further react to produce more stable 2,2'-dihydroxybenzophenone moieties.
UV irradiation also affects the extent of crystallization in PC. The extent of crystallization was monitored via density measurements. The PC density data, [[rho].sub.s], followed the equation, Eq. 1 as
[[rho].sub.s] = [w.sub.s][[rho].sub.1]/[w.sub.s] - [B.sub.s] (1)
where [w.sub.s], [[rho].sub.1] and [B.sub.s] are the sample weight in air, density of liquid (deionized water), and sample weight in liquid, respectively. Furthermore, the volume fraction crystallinity, [X.sub.c], follows the rule of mixtures, that is,
[X.sub.c] = ([[rho].sub.s] - [[rho].sub.A])/([[rho].sub.c] - [[rho].sub.A]) (2)
where [[rho].sub.A] and [[rho].sub.c] are the densities of perfectly amorphous phase and perfectly crystalline phase, respectively. The densities of crystalline and amorphous phases are 1.307 and 1.188 g/[cm.sup.3], respectively . The densities of PC irradiated with different UV doses are listed in Table 3. Using Eq. 2, one obtains the [X.sub.c] values for PC at different doses shown in Table 3. The crystallinity increased linearly with increasing UV dose.
The glass transition temperatures of UV-irradiated PC before and after acetone transport are listed in Table 2. It can be seen that the glass transition temperature of PC after acetone treatment is lower than that before acetone treatment. Furthermore, the glass transition temperature of UV-irradiated PC immersed in acetone is always higher than the maximum immersing temperature (30[degrees]C) in this study. Note that PC samples changed from transparent to white and opaque immediately when immersed in acetone. Figure 2a-d shows the acetone sorption curves for PC irradiated to different UV doses at temperatures ranging from -23[degrees]C to 30[degrees]C. The transport behavior is characterized by rapid uptake at the short times followed by a slower sorption rate until reaching a steady state. Generally, the time to reach the steady state decreases with increasing temperature for both UV-irradiated and nonirradiated PC. It can be seen from Fig. 2 that the data for all doses show regions where sorption is much less affected by temperature than that noted in other regions. The analysis that follows uses a model to better understand this behavior.
The data in Fig. 2 are curve-fitted by the model proposed by Harmon et al. . The polymer is initially assumed to be solvent-free, and the concentration of acetone is constant on both outer surfaces at all times. Acetone transport in the PC is characterized by Case I, Case II, and anomalous sorption. The parameters D and v correspond to diffusion coefficient and velocity for Case I and Case II transport, respectively. The mass gain in PC is calculated from 
[mathematical expression not reproducible] (3)
[[lambda].sub.n] = (vl/2D)tan [[lambda].sub.n] (4)
[[beta].sub.n.sup.2] = [[lambda].sub.n.sup.2] + [v.sup.2][l.sup.2]/4[D.sup.4] (5)
[M.sub.[infinity]] is the equilibrium mass of acetone and 2l is the thickness of the specimen. [[lambda].sub.n] is the positive nth root of Eq. 4. The solid lines in Fig. 2 are obtained using Eq. 3 with parameters D and v shown in Fig. 3a and b, respectively. It can be seen from Fig. 2 that the experimental data are fitted with the theoretical prediction. Because the mass uptake data are almost overlapped in the temperature range above -2[degrees]C for all doses (Fig. 2), D and v data are expected to overlap as well. The D and v data shown in Fig. 3a and b are divided into three regions. The data in each region are well fitted by straight lines. That is, both D and v satisfy the Arrhenius equation. The activation energy for acetone diffusion (Fig. 3a) above -2[degrees]C is 24.4 kJ/mol for UV doses from 0 to 211.7 J/[cm.sup.2] (region I). In the lower temperature range, the data splits into a region, where D is independent of temperature and then into 4 separate linear regions. The activation energy for the nonirradiated PC is 46 kJ/mol, and constant at 50 kJ/mol for all doses from 90.7 to 211.7 J/[cm.sup.2]. Similarly, the activation energy for Case II transport in PC (Fig. 3b) is 2.8 kJ/mol in temperature range above -2[degrees]C and 5.2 kJ/mol at lower temperatures for irradiated samples and controls. The transport transition temperatures for acetone in PC (the temperatures on the horizontal line) are - 2[degrees]C, - 8[degrees]C, - 13[degrees]C, and - 18[degrees]C for UV doses 0, 90.7, 151.2, and 211.7 J/[cm.sup.2], respectively. This phenomenon is similar to that observed in metals when phase changes occur. For example, a transport transition temperature was observed at 863[degrees]C when zirconium underwent phase transformation from the bcc beta phase to the hep alpha phase . Also, a Curie temperature (770[degrees]C) was observed for self-diffusion in iron when the paramagnetic-ferromagnetic transition occurred , It is worthy to mention that the value of v in Fig. 3b is always positive, suggesting that the velocity direction for Case II transport is from the center to the outer surface. This direction is the opposite to that of Case I transport in amorphous polymer systems. The reason for the positive v in this study is that the solvent induces crystallization during mass transport. The newly formed crystals "squeeze" the solvent from the matrix. Solvent-induced crystallization is discussed in the following section. These phenomena were also observed by Wu et al. , who studied acetone transport in gamma-irradiated PC where similar plots depicted transition temperatures. However, the effect of gamma irradiation on energy barriers of both Case I and Case II transport is more pronounced than the effect of UV irradiation over the dose ranges investigated. Earlier Kambour et al.  measured acetone sorption in thin films of PC with a quartz spring balance at different vapor pressures. They found that above a certain partial pressure when the maximum weigh, Wmax, was obtained, if the sample was allowed to rest at that pressure, an onset of desorption and whitening of the sample occurred. The time for the desorption onset decreased as the partial pressure increased. This was attributed to crystallization. At high partial pressure [W.sub.max], was never reached. As we are using liquid acetone, we believe that we are at the point where [W.sub.max] is not reached.
Equilibrium Swelling Ratio
The equilibrium swelling ratio 5 is defined as the mass of saturated solvent at a given temperature divided by the specimen mass. The equilibrium swelling ratios of acetone in UV-irradiated PC shown in Fig. 4 clearly satisfy the van't Hoff equation for different doses and clearly exhibit transition temperatures. S values decreased with increasing temperature, implying that acetone transport in PC is an exothermic process. The effect of UV dose on S is more pronounced in the lower temperature range. The equilibrium swelling ratios decreased with increasing dose in the low temperature range. The plots of S versus 1/T were almost parallel for all doses in low temperature range and their heats of mixing were - 27.8 kJ/mol. The transition temperatures [T.sub.s]s at the slope change in Fig. 4 ranged from - 2[degrees]C to - 18[degrees]C and decreased as the dose increased. The shift in [T.sub.s] is likely influenced by molecular weight and changes in the populations of different crystalline forms, as discussed later. The PC crystallizes more easily and has more well developed spherulites at lower molecular weights as shown in Table 3. It can be seen from Table 1 that the molecular weight decreased with increasing UV dose. The negative heat of mixing is accounted partially for by the formation of spherulites because crystallization is an exothermic process. Again the acetone swelling and heat of mixing in gamma-irradiated PC exhibited similar behavior to that of UV irradiation .
Figure 5a-c shows the polarized light microscopic images of cross-sections of PC swollen by acetone for 60 min at different temperatures. The acetone moved from the right-hand side to the left-hand side. These pictures illustrated three regions in the specimen: (a) the unswollen core at the left side appears fuzzy and white, (b) the swollen zone near the unswollen core is featureless and black, and (c) the crystallization zone located between two black regions consists of many spherulites. Note that the right-side black region was filled with air. Thus a free surface was situated at the boundary between the crystallization layer and right-side air region; a crystallization front was located at the interface between the crystallization layer and the swollen zone; and the diffusion front was defined as the line between the swollen zone and unswollen core. The diffusion front is very sharp. Sharp fronts are common and are observed when solvent of low molecular weight penetrates a glassy matrix. The test temperature is below the glass transition temperature in the unswollen region. A diffusion front occurs when the displacement of the sharp front is proportional to the square root of transport time. The diffusion front is easily detected from microscope image because the refractive index of the glassy core is different from that of swollen zone. The diffusion front is always ahead of crystallization front because solvent-induced crystallization is required to overcome the energy barrier for polymer chain arrangements transitioning from the amorphous to the crystalline state. The incubation time for solvent-induced crystallization is similar to that of thermally induced crystallization .
As shown in Fig. 5a, no spherulites were observed at--19[degrees]C, implying that the incubation time for crystallization is longer than 60 min at this temperature. The crystallization zone at 20[degrees]C was greater than that at 0[degrees]C as shown in Fig. 5b and c. This indicates that the incubation time for crystallization increases with decreasing temperature, whereas crystallization zone increases with increasing temperature. Stress relaxation arising from solvent transport becomes more complicated in PC than in amorphous polymers such as poly(methyl methacrylate). The density of the crystallization region (1.188 g/[cm.sup.3]) is lower than that of the amorphous zone (1.307 g/[cm.sup.3]), resulting in Case II transport of solvent from center to the outer surface of the PC, that is, v is positive. However, the crystallization front is not easy to measure quantitatively and needs further attention in the near future.
The distance X from the free surface to the sharp front as a function of square root time [t.sup.1/2] is shown in Fig. 6a-d for UV-irradiated PC with various doses at temperature--23[degrees]C ~ 20[degrees]C. The data were fitted with Eq. 6 proposed by Lapcik et al.  as
[X.sup.2] = 2 [D.sub.f]t (6)
where [D.sub.f] is the diffusion coefficient for the diffusion front. [D.sub.f] values obtained from Fig. 6 are listed in Table 4 and satisfy the Arrhenius equation. The behavior of [D.sub.f] with respect to temperature is similar to those of the D and v shown in Fig. 3. The transition temperatures ([T.sub.f]) at the slope change reveal that [T.sub.f]s decrease as the dose increases from 0 to 211.7 J/[cm.sup.2]. [D.sub.f] values are nearly identical for all doses at temperatures above--2[degrees]C. In this region, the activation energy is 18.8 kJ/mol for all samples. However, at a given temperature below [T.sub.f], [D.sub.f] values increase with increasing dose and the activation energies vary only slightly from 51.8 to 52.4 kJ/mol for irradiated and unirradiated samples. These results are similar to those obtained by Wu et al. for gamma radiation , where activation energies for [D.sub.f] were 16.7 kJ/mol in the high temperature range and 62.7 kJ/mol in the low temperature range for all doses. When [D.sub.f] values from front measurements are compared to diffusion coefficients from mass transport, the activation energy from [D.sub.f] is lower than that of D in the high temperature range for all UV doses; activation energy for [D.sub.f] is slightly higher than that of D in the low temperature range.
The chemical bonds in PC involved in acetone transport were analyzed using FTIR spectroscopy. The IR absorbance traces of PC before and after immersion in acetone, and PC desorbed in air for 1 week are shown in Fig. 7a. Before acetone uptake, PC exhibits high intensity peaks at 1771 [cm.sup.-1] for the C=O stretching mode and 1503, 887, and 767 [cm.sup.-1] corresponding to aromatic C=C stretching mode, in-plane vibration of carbonate group, and out-of-plane vibration of carbonate group, respectively, as shown by the solid line in the lower curve in Fig. 7a. The results are compatible with those obtained by Apai and McKenna  for pure PC. Acetone exhibits peaks at 1710 [cm.sup.-1] for the C=O stretching mode, 1364 [cm.sup.-1] for the C-H deformation mode, and 1222 [cm.sup.-1] for the C--O stretching mode, respectively, as shown by the dashed line in lower curve in Fig. 7a. In comparing the middle curve and lower curve in Fig. 7a, it is noted that the IR intensity in PC saturated with acetone is equal to the superposition of intensities for pure PC and pure acetone. Furthermore, after desorption in an ambient environment for 1 week, the acetone evaporated from the specimen and the IR peaks corresponding to acetone disappear (peaks at 1710, 1364, and 1222 [cm.sup.-1]). It is worthy to mention that the peak position shifts from 1771 [cm.sup.-1] for specimen before acetone absorption to 1764 [cm.sup.-1] for the specimen after acetone absorption and specimen after desorption for 1 week. This is shown in Fig. 7b by solid and chain lines, respectively. The position shift arises from solvent-induced crystallization , implying that the crystallization is an irreversible process .
In summary, the acetone transport in PC is a physical phenomenon, not a chemical reaction. Our transport model is sensitive to changes in the glass transition temperature, amorphous to crystalline transitions and free volume changes. This confirms earlier transport studies that detected physical matrix changes in polymers deformed by mechanical methods [20, 21, 44].
The transport transition temperature may also be explained using FTIR spectroscopy. Acetone has two types of tautomers, keto and enol forms. The keto-enol tautomerization can be represented as followings:
C[H.sub.3]-CO-C[H.sub.2] [left and right arrow] C[H.sub.3]-COH=C[H.sub.2] (7)
The equilibrium at room temperature favors the formations of keto form. The amount of enol at equilibrium depends on temperature. Figure 8 shows the IR transmittance of PC immersed in acetone at different temperatures. The thickness of each sample is 0.2 mm and the swelling ratio of acetone in sample is ~20%. The peaks at 1710 and 3400 [cm.sup.-1] correspond to C=O stretching bonds and O-H bond, respectively. The transmittance of C=O stretching bond increased obviously with decreasing mass up-take temperature, implying that the absorption peak intensity of C=O stretching bond increased with increasing mass up-take temperature. According to Eq. 7, C=O stretching bond corresponds to the keto form. That is, the amount of keto form of acetone in PC increased with increasing mass up-take temperature. The absorbance intensity of O-H bond obtained from the transmittance is shown in the inset in which the absorbance of O-H bond increased with decreasing mass-uptake temperature. Compared the inset of Fig. 8 and Eq. 7, O-H bond corresponds to the enol form and the amount of enol form of acetone in PC increased with decreasing mass-uptake temperature. Based on the above statement, acetone transport in PC favors the keto form at high temperatures and the enol form at low temperatures, respectively, but the amounts of enol and keto forms in PC are unknown. Although compared to enol form, keto form still dominates at low temperature range, the transport transition temperature may be partially affected by enol form in PC. In the study, the transport transition temperatures are--2[degrees]C,--8[degrees]C,--13[degrees]C, and--18[degrees]C for UV doses of 0, 90.7, 151.2, and 211.7 J/[cm.sup.2], respectively.
XRD Diffraction Pattern
The transport transition temperature is also explained using X-ray diffraction. Figure 9 shows diffraction angles, 2[theta], of PC immersed in acetone at--10[degrees]C and +30[degrees]C, respectively. From Fig. 9, we calculated the interplanar spacing, d, and crystal plane (hkl) corresponding crystalline structure. The results are listed in Table 5. When T = 30[degrees]C, PC XRD revealed two peaks at 2[theta] = 17.3[degrees] and 25.7[degrees], which correspond to the orthorhombic structure with a =12.1 [Angstrom], b = 10.1 [Angstrom], c = 21.5 [Angstrom]; while T = -10[degrees]C, PC XRD data revealed peaks at 2[theta] = 17.0[degrees], 25.5[degrees], and 33.6[degrees] corresponding to the orthorhombic structure and 8.4[degrees], 29.8[degrees], and 32.2[degrees] corresponding to the monoclinic structure with a = 12.3 [Angstrom], b = 10.1 [Angstrom], c = 20.8 [Angstrom], and [gamma] = 84[degrees]. In addition to the orthorhombic structure, the monoclinic structure also appeared in PC immersed in acetone at--10[degrees]C below the mass transport transition temperature. This is similar to data obtained for the transport transition temperature of self-diffusion of [beta]-zirconium which was determined to be due to a phase change . Furthermore, the spacing between two adjacent (hkl) in orthorhombic and monoclinic structures obtained by Radhakrishnan et al. , who studied PC synthesized by solid state polycondensation, are also listed in Table 5. Compared to the present values of interplanar spacing to that of Ref. 45, the maximal difference is 2.3% at plane (124) of orthorhombic structure. The axes of orthorhombic structure reported by Radhakrishnan et al.  were a = 12.1 [Angstrom], b = 10.1 [Angstrom], c = 22.0 [Angstrom], which were almost the same as ours. XRD data leads us to believe that the transition temperatures observed in this research may be due to a phase change from rhombohedral to partial monoclinic lattices. This is similar to the data obtained for the transport transition temperature of self-diffusion of [beta]-zirconium which was determined to be due to a phase change .
Comparison Between As-Received and Purified PC
To understand the effect of additives on the absorption of PC, [sup.1]H MNR spectra of as-received and purified PCs were obtained by a Bruker AVANCE-500 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) using CDC13 solvent. Figure 10 shows the [sup.1] NMR spectrum of the as-received PC and purified PC. A number of characteristic bands were assigned and listed in Table 6. The peak codes a, b, c, d, e, and f shown in Fig. 10 can be found the corresponding bonds of PC shown in the inset of figure. The [sup.1] NMR spectrum of purified PC is the same as that of as-received PC. According to Table 6, the present data are compatible with those obtained from Ref. 46. Peak positions at 7.25 and 1.55 ppm correspond to CD[Cl.sub.3] solvent and residual water, respectively . Furthermore, the glass transition temperatures of as-received and purified PC obtained from DSC curves were 151.3[degrees]C and 150.7[degrees]C, respectively. The above evidences illustrate that the effect of additives on acetone absorption behavior in PC is insignificant.
Acetone transport in UV-irradiated PC was investigated. Important conclusions are described in the following:
1. The glass transition temperature and molecular weight of PC decrease with increasing UV dose, while crystallinity increases.
2. Acetone transport in UV-irradiated PC is anomalous. The direction of Case II transport is opposite to that of Case I diffusion that is due to solvent-induced crystallization.
3. Based on mass uptake and diffusion front studies, the transport transition temperatures are estimated as -2[degrees]C, -8[degrees]C, -13[degrees]C, and--18[degrees]C for UV doses of 0, 90.7, 151.2, and 211.7 J/[cm.sup.2], respectively.
4. The mass uptake decreases with increasing UV in low temperature range, whereas it is nearly identical same for all UV dose at temperatures above the transition temperature.
5. The activation energies for Case I and Case II transport in PC are 24.4 and 2.8 kJ/mol, respectively, in the high-temperature range for all doses. In the low-temperature region, activation energies for Case I transport in PC are 50 kJ/mol for non-irradiated samples and 46 kJ/mol at UV dose ranging from 90.7 to 211.7 J/[cm.sup.2]. The activation energy for Case II transport is 5.2 kJ/mol for all doses in the low-temperature range.
6. The acetone transport in UV-irradiated PC is an exothermic process. The heat of mixing is -3.8 and -27.8 kJ/mol in the temperature range above and below the transport transition temperature, respectively. The equilibrium swelling ratio decreases with increasing UV dose in low-temperature range, but that is nearly the same for all UV doses in high-temperature range.
7. XRD data lead us to believe that the transition temperatures observed in this research may be due to a phase change from rhombohedral to partial monoclinic lattices.
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Yu Chun Hsiao, (1) Julie P Harmon, (2) Yu-Fan Chuang, (1) Donyau Chiang, (3) Sanboh Lee [iD] (1)
(1) Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
(2) Chemistry Department, University of South Florida, Tampa, Florida 33620
(3) Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, 30076, Taiwan
Correspondence to: S. Lee; e-mail: email@example.com Contract grant sponsor: Ministry of Science and Technology, Taiwan.
Published online in Wiley Online Library (wileyonlinelibrary.com).
Caption: FIG. 1. IR spectra of polycarbonate irradiated with different doses. [Color AQ figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 2. Acetone absorption in irradiated PC: (a) 0, (b) 90.7, (c) 151.2, and (d) 211.7 J/[cm.sup.2]. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 3. Arrhenius plots of (a) d and (b) positive v for different UV doses.
Caption: FIG. 4. Van't Hoff plots of equilibrium swelling ratio in UV-irradiated PC with various doses.
Caption: FIG. 5. Microscopic image of PC swollen by acetone at (a)--23[degrees]C, (b) 0[degrees]C, and (c) 20[degrees]C for 60 min. The arrow indicates the direction of acetone transport.
Caption: FIG. 6. Distance from the surface to the diffusion front as a function of time for various UV doses: (a) 0, (b) 90.7, (c) 151.2, and (d) 211.7 J/[cm.sup.2], respectively.
Caption: FIG. 7. (a) IR spectra of acetone, PC, PC with saturated acetone, and PC after desorption in an ambient environment for 1 week, (b) Compare IR spectra of PC before absorption to that after absorption. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 8. IR transmittance of PC immersed in acetone at different temperatures. The inset shows the absorbance of PC immersed in acetone at different temperatures in the range from 3250 to 3750 [cm.sup.-1]. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 9. X-ray diffraction patterns of PC immersed in acetone at temperatures 30[degrees]C and--10[degrees]C. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 10. [sup.1]H NMR spectra of PC.
TABLE 1. Weight-average molecular weight and number-average molecular weight of PC irradiated with different UV doses. Dose 0 90.7 151.2 211.7 (J/[cm.sup.2]) [M.sub.w] 61,517 58,839 56,964 56,250 [M.sub.n] 34,642 25,714 25,625 23,661 TABLE 2. Glass transition temperatures [T.sub.g]s of UV-irradiated PC with and without acetone absorption. UV dose Acetone [T.sub.g] of PC [T.sub.g] (J/[cm.sup.2]) absorption (%) with acetone of pure PC ([degrees]C) ([degrees]C) 0 31.1 68.0 150.7 90.7 24.0 70.0 148.3 151.2 25.7 67.7 147.0 211.7 28.1 66.3 146.1 TABLE 3. Density and volume fraction crystallinity of PC irradiated with different UV doses. UV dose Density Crystallinity (%) (J/[cm.sup.2]) (g/[cm.sup.3]) 0 1.188 0 90.7 1.191 [+ or -] 0.003 2.52 [+ or -] 2.52 151.2 1.194 [+ or -] 0.006 5.04 [+ or -] 5.04 211.7 1.197 [+ or -] 0.007 7.56 [+ or -] 5.88 TABLE 4. Diffusion constant, [D.sub.f] ([10.sup.-8] [cm.sup.2]/s), of the sharp front of acetone in UV-irradiated PC at different temperatures. 0 90.7 (J/[cm.sup.2]) (J/[cm.sup.2]) -23[degrees]C 1.88 3.09 -18[degrees] 3.74 5.52 -13[degrees] 6.27 7.18 -8[degrees] 8.85 13.6 -2[degrees] 13.2 13.6 0[degrees]C 16.4 16.1 5[degrees]C 18.9 17.6 20[degrees]C 26.5 29.2 151.2 211.7 (J/[cm.sup.2]) (J/[cm.sup.2]) -23[degrees]C 4.84 8.12 -18[degrees] 7.14 13.2 -13[degrees] 12.8 13.2 -8[degrees] 12.8 13.2 -2[degrees] 12.8 13.2 0[degrees]C 16.9 17.5 5[degrees]C 17.7 19.8 20[degrees]C 26.3 27.9 TABLE 5. The diffraction angle 29, interplanar spacing d([Angstrom])and crystalline plane (hkl) of PC with saturated acetone solvent at +30[degrees]C and - 10[degrees]C. T 2[theta] d d ([degrees]C) ([Angstrom]) ([Angstrom]) (45) 30[degrees]C 17.3[degrees] 5.13 5.19 25.7[degrees] 3.47 3.55 -10[degrees]C 8.4[degrees] 10.52 10.54 17.0[degrees] 5.21 5.19 25.5[degrees] 3.49 3.55 29.8[degrees] 3.00 3.05 32.2[degrees] 2.78 2.79 33.6[degrees] 2.67 2.65 T 2[theta] (hkl) Crystal structure ([degrees]C) 30[degrees]C 17.3[degrees] 210 Orthorhombic 25.7[degrees] 124 Orthorhombic -10[degrees]C 8.4[degrees] 101 Monoclinic 17.0[degrees] 210 Orthorhombic 25.5[degrees] 124 Orthorhombic 29.8[degrees] 400 Monoclinic 32.2[degrees] 230 Monoclinic 33.6[degrees] 404 Orthorhombic TABLE 6. The comparison of characteristic NMR peak positons (ppm) of as-received PC, purified PC, and data obtained from Ref. 46. Peak code As-received PC Ref. 46 PC precipitate a -- -- 6.91 b 7.19 7.19 7.19 c 7.27 7.27 7.27 d 7.20 7.20 7.21 e 1.72 1.72 1.72 f 1.36 1.36 1.32
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|Author:||Hsiao, Yu Chun; Harmon, Julie P.; Chuang, Yu-Fan; Chiang, Donyau; Lee, Sanboh|
|Publication:||Polymer Engineering and Science|
|Date:||Jul 1, 2018|
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