Miscible blends of fluorocarbon copolymer with acrylic copolymer and poly(vinyl acetate), PVAc: new alternatives to low refractive index cladding materials

Nuclear Instruments andMethods in Physics Research B69 (1992) 332-340 North-Holland

HUN B

Seam Iateracttoas withMaterials&Atoms

Miscible blends of fluorocarbon copolymer with acrylic copolymer and poly(vinyl acetate), PVAc: new alternatives to low refractive index cladding materials Justin Gaynor, Verlyn Fischer, James K . Walker and Julie P. Harmon Department of Physics, University of Florida, Gainesville, FL 32611, USA Received 25 March 1991 and in revised form 18 November 1991 A fluorocarbon copolymer (Kynar 7201) was solvent blended and melt blended with acrylic copolymer and PVAc . This opaque Kynar copolymer exhibited enhanced transparency when blended with PMMA and PVAc over a range of concentrations. Transparency  low refractive indices and low temperature processibility make these blends novel alternatives to conventional cladding materials used for plastic scintillating fibers . 1. Introduction Commercially available scintillating fibers have polystyrene (PS) or polyvinyltoluene (PVT) cores with refractive indices of 1.59. These core materials are often clad with polymethyl methacrylate (PMMA), which has a refractive index of 1.49. Neither PS nor PVT is optically stable in high-radiation environments . We have identified an optically radiation stable polydiorganosiloxane core material with a refractive index of 1.54 [1,2]~ This siloxane material requires a cladding material with a lower refractive index than PMMA in order to conduct light efficiently . Polymers such as perfluorinated alkoxycopolymers and perfluorinated ethylene-propylene copolymers have refractive indices of 1.34 and 1.35, respectively [3]. These copolymers have limited use as cladding materials for plastic scintillator fibers; they are semicrystalline and opaque and therefore lead to high light scattering bosses . In addition, they must be processed at temperatures greater than 300°C. At these high temperatures, the fluors in the scintillator core will degrade. This means that they cannot be used in production of fiber by coextrusion processes. The purpose here is to identify low refractive index, transparent copolymer blends which can be used to clad fibers when the refractive index of the core material is 1.54. In the past, experiments have shown that polyvinylid'ene fluoride, PVFZ, can be blended with Correspondence to. Dr . J.P. Harmon, Department of Physics, University of Florida, 215 Williamson Hall, Gainesville, FL 32611, USA .

PMMA and PVAc to produce miscible materials [4-7]. When crystallinity is absent, these blends are transparent. It was concluded that the carbonyl group present in the PVAc and PMMA is responsible for interacting with PVFZ to induce miscibility. Here, we report the use of a fluorocarbon copolymer, Kynar7201, in blends with acrylic copolymer, CP-41, and PVAc . This Kynar copolymer contains PVFZ combined with polytetrafluoroethylene, PTFE . We characterized a series of blends to identify miscible blends which did not crystallize, diminishing transparency. 2. Experimental Kynar 7201 was obtained from Penwalt Corporation in Philadelphia, PA . It is a copolymer of PVFZ and PTFE . CP-41, was obtained from Continental Polymers . It is a copolymer of methyl methacrylate and 10% ethyl acrylate. PVAc was purchased from Aldrich Chemical Company. Samples were prepared by heat and solvent blending. Solvent blended polymers were mixed and dissolved in acetone. 0.5 mm films were made by evaporating the solvent and pressing the material between polished metal plates in a hot press. These thick films were then placed between two plates of quartz glass with drops of PDMS oil providing optical contact for transmission measurements. Materials were heat blended in a CSI mini extruder and spun into fibers. 1 cm thick disks were prepared by hot pressing extruded fiber between glass disks in a piston mold and allowing slow cooling.

0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved

J. Gaynor et al. /Miscible blends offluorocarbon copolymer with acrylic copolymer and PVAc

333

io0

z 0 H N N S N Z Q r K

50

0 190

390

WAVELENGTH (nm)

590

790

Fig. 1 . Transmission spectra of 0.5 mm thick films of Kynar 7201 blendedwith CP-41. Numbers refer to wt.% Kynar.

Absorption spectra were recorded on a HewlettPackard model 8452A diode-array spectrophotometer. Refractive indices were measured on an Abbè refractometer obtained from Fisher. A DuPont differential scanning calorimeter, model 710, was used to determine glass transition temperatures, melting temperatures, and the heats of transition . Samples were scanned at a rate of 10°C/min. The molecularweight of PVAc

in acetone was determined with a Cannon-Fenske size 50 viscometer obtained from Fisher Scientific. The viscometry was carried out in an oil bath which was thermally stabilized at 25°C with a model 9000 isotemp refrigerated circulator from Fisher . Samples were irradiated in air with a "Co gamma source at a dose rate of 41 .4 krad/h . Thecharacteristic gamma energies from O'Co are 1.17 and 1.33 MeV.

aH

390

WAVELENGTH

tnm)

590

790

Fig. 2. Transmission spectra of 0.5 mm thick films of Kynar7201 blended with PVAc. Numbers refer to wt.% Kynar.

334

J. Gaynor et al. / Miscible blends offluorocarbon copolymer with acrylic copolymer andPVAc

3. Results and discussion 3.1. Molecular weight

Dilute-solution viscometry measurements carried out in acetone at 25°C with five different concentrations yielded a viscosity average molecular weight of 103000 for PVAc . The molecular weight was calculated from intrinsic viscosity molecular weight constants obtained from Kurata [10]. These constants are not available for Kynar 7201 . However, miscibility results reported here are specific to these polymers, since the mutual solubility of two polymers decreases as the molecular weight of either polymer increases [11] .

20010 d . r 0 N

F 10000 d v c

a

0

3.2. Transparency

All acetone solutions of Kynar 7201 with CP-41 and PVAc were transparent . In the past, it was shown that solutions of PVFZ and poly(vinyl methyl ether), PVME, in dimethylformamide, DMF, phase separated even though both polymers are soluble in DMF [7]. Solutions of PVFZ and poly(vinyl methyl ketone), PVMK, in DMF were clear. PVME was found to be immiscible with PVFZ whereas PVMK is miscible with PVFZ. Clear solutions, then, are an indication of possible compatibility . Films of Kynar 7201 blends with CP-41 and PVAc were heated to temperatures greater than 170°C. All films became clear when heated above the melting point of pure Kynar 7201 (120°C). Similarly, PVFZ/ PVMK films became clear when heated above the melting point of PVFZ. This, again, is an indication of polymer compatibility and will be discussed more thoroughly in the thermal analysis section. The transmission spectra of 0.5 mm films compression molded from Kynar 7201 solvent blended with CP-41 are shown in fig. 1. The Kynar concentration was varied from 0% to 100% in increments of 25% by weight. At 400 nm there is 91% transmission through the 50/51) blend compared with 3% transmission in pure Kynar 7201 . Solution blends made with PVAc are less transparent than those made with CP-41 . This is depicted in fig. 2. The transmission spectra of the 25% Kynar 7201 and the 50% Kynar 7201 were nearly identical. Transparency is one indication of true molecular compatibility [12] . Fig. 3a plots the area under the transmission curve vs the weight percent Kynar in CP-41 for 400-700 nm light . 0%, 25%, and 50% Kynar blends are equally transparent . Fig. 3b is an identical plot for Kynar/PVAc blends. Transparency diminishes as the Kynar concentration increases . The losses in transmission depicted in figs. 3a and 3b are attributed to the formation of crystalline regions in the blends.

20

0

40 60 Weight% Kynar

so

100

20000

c 0

F

c dP

0

0

20

40 60 Weight% Kynar

so

100

Fig. 3. (a) Area under transmission curve vs wt .% Kynar blended with CP-41 . Spectra from 400-71)0 nm . (b) Area under transmission curve vs wt.% Kynar blended with PVAc . Spectra from 400-700 nm. This is verified by DSC results which will be discussed later. Crystallinity is deterred by quenching from the melt. With this in mind, a blend of 50/50 Kynar/PVAc was prepared by melt blending using a CSI mini extruder with a fiber die. This processing method produces material similar to that produced during bicomponent fiber spinning, wherein the cladding material is spun around the core material in a one-step process. The material cools rapidly upon emerging from the extruder . The blend was considerably clearer than the material made by hot pressing . Rapid cooling impedes crystallization and allows the material to remain relatively transparent. This extruded material was heated to 170°C in a press and slow cooled. After slow cooling, crystallization rendered the material opaque .

!. Gaynor et al. / Miscible blends of fluorocarbon copolymer with acrylic copolymer andPVAc

335

Table 1 Summary of differential scanning calorimetry data for selected samples Material

Tg [°C]

Tm (°C]

PTFE PVFZ

117 ' -39 b

327 171 b

Heat of transition V/91

CP-41/Kynar 7201 blends Wt.% Kynar: 100% (0 .5 mm film) 75% (0 .5 mm film) 50% (0 .5 mm film) 25% (0.5 mm film) 0% (0.5 mm film)

8.2 24.0 47.6 58.7 87 .7

123 116

43 .2 23 .1

PVAc/Kynar 7201 blends Wt .% Kynar: 100% (0.5 mm film) 75% (0.5 mm film) 50% (0.5 mm film) 25% (0.5 mm film) 0% (0.5 mm film)

8 .2 9.1 17.1 27.9 31 .5

123 117 108 105

43.2 28.1 13 .9 3 .8

' Ref. [8]. b Ref. [9].

3.3. Differential scanning calorimetry Differential scanning calorimetry data for selected samples are summarized in table 1 . Kynar 7201 exhibited a single glass transition at 8 .2°C . A melting endotherm was noted at 123°C ; its energy of transition was calculated as 43.2 J/g. This melting temperature

too 80 C

F

(T.) is lower than that of homopolymers of PVFZ and PFTE. The low melting point of this Kynar very likely results from the fact that the copolymer forms less perfect crystals than the more stereoregular homopolymers. Poly(vinyl acetate) exhibited a glass transition at 31 .5°C. The location of this transition agrees with re-

40

i

30

60 i i

20

40 i

10

20 i

0 0 20 40 60 . 80 ' 100 100 40 60 80 Percent Kynar 7201 Weight Percent Kynar 7201 Fig . 4. (a) Glass transition temperature vs wt .% Kynar blended with CP-41 . (b) Glass transition temperature vs wt .% Kynar blended with PVAc. 0

0

20

336

J. Gaynor et al. / Miscible blends offluorocarbon copolymerwith acrylic copolymer and PVAc 100

z N N M

N â

50

x

190

390

Fig. 5. 0.5 mm filmsof CP-41 before

ported values. The glass transition

WAVELENGTH (nm)

590

790

andafter(---) 10 Mrad gamma irradiation in air .

-)

temperature of

sharpness of the DSC trace diminished as the Kynar

CP-41 was 87.7°C. CP-41 is a low-viscosity type designed for maximum flow rates during processing .

concentration increased . The 75% Kynar/PVAc blend showed no discernable glass transition. Plots of the Tg

Kynar/CP-41 and Kynar/PVAc blends exhibited a single glass transition temperatures which decreased with Kynar content. The presence of a single glass transition temperature is a widely accepted test for miscibility [131 . The glass transition temperatures of CP-41 and PVAc are different enough from that of

Kynar 7201 to render these results unambiguous . The

vs wt.% Kynar are shown in figs . 4a and 4b. These plots exhibit a peculiar curvature as has been not.-d in the past for PVFZ blends with PVAc [5] and PVMK [7]. The location of the melting endotherm for Kynar

7201 decreased by 7°C from 100% Kynar to 75% Kynar in the CP-41 blend. This compares to a 4.5°C decrease noted in PVFZ/PMMA blends over the same concen-

100

z 0 M N N M

50

r x

0 WAVELENGTH (nm)

Fig. 6. 0.5 mm films of 25 wt .% Kynar 7201 in CP-41 before (

)and after (---)10 Mrad gamma irradiation in air.

J. Gaynor et al. / Miscible blends offluorocarbon copolymer with acrylic copolymer and PVAc

WAVELENGTH (nm)

Fig. 7. 0.5 mm films of 50 wt .% Kynar 7201 in CP-41 before ( tration range [141. The 25% and 50% Kynar blends showed no evidence of a melting endotherm. Similar results were noted for the PVFZ/PMMA blends [141, where it was concluded that the polymer pairs were miscible up to PVFZ concentrations where crystallinity was noted. In crystalline blends there is a two-phase structure. Crystalline regions contain pure PVFZ, while the amorphous region is a compatible mixture of PMMA and PVFZ. This mixture contains more PMMA than the total blend due to a loss of PVFZ to crystal-

590

) and after (---)10 Mrad gammairradiation in air. lization. The Tm depression in the PMMA blends is thought to be due to less perfect crystalline regions developing in blends. All of the PVAc blends showed a melting transition . There is a 15° decrease in Tm as the Kynar concentration is decreased from 100% to 50%. Bernstein et al . [51 noted a 15° depression in PVFZ/PVAc blends over the same concentration range. The melting endotherms of the Kynar/PVAc blends decrease with Kynar content. Other blend systemsshow similar trends

WAVELENGTH (nm)

Fig. 8. 0.5 mm film of 75 wt .% Kynar7201 in CP-41 before (

337

590

790

)and after (---)10 Mrad gamma irradiation in air.

33 8

J. Gaynor et al. / Miscible blends of fluorocarbon copolymer with acrylic copolymer and PVAc 100

z 0 H N N N N z a Q H x

WAVELENGTH Fig. 9. 0.5 mm film of Kynar7201 before (

(nm)

) andafter(---) 10 Mrad gammairradiation in air.

[5]. The effect is thought to be due to retardation of the crystallization rate by the second polymer component . The quenched extruded fiber containing 50 wt .% PVAc in Kynar 7201 had a melting endotherm magnitude of 6.2 J/g. This is compared with the endotherm of an extruded fiber which was heated above its Tm and slow cooled and showed an endotherm magnitude of 12.3 J/g. Visual observation of increased clarity in

tion . The CP-41 blends, however, have a much lower tendency to crystallize . It is more reliable to concentrate on the CP-41/Kynar system in producing clear blends for fiber cladding. 3.4. Optical radiation stability

the quenched sample supported these DSC results,

Most scintillator fibers are currently clad with PMMA. We compare the optical radiation stability of CP-41 with Kynar 7201 blends in figs. 5-9. The trans-

which showed that quenching diminishes crystalliza-

mission spectra were recorded before and after 0.5 mm

100

Z N M

L

0 390

Fig. 10. 0.5 mm film of PVAc before (

390

WAVELENGTH

(nm)

590

790

) andafter(---) 10 Mrad gamma irradiation in air.

339

J. Gaynor et al. / Miscible blends offluorocarbon copolymer with acrylic copolymer and PVAc

thick films were exposed to 10 Mrad from a 6°Co gamma source in an air environment. The pure CP-41 film exhibited the highest amount of radiation damage ; the blends, however, also exhibit observable radiation damage even in these thin films . From this we can conclude that Kynar blends are at least slightly more optically stable than CP-41 in the presence of ionizing radiation . In the samples which exhibited crystallinity, an increase in transmission was observed after irradiation . We believe this is due to the decrease in crystallinity which often accompanies high-energy irradiation [15]. Because there is some crystallinity in all of the Kynar/PVAc blends, we observed a slight increase in the transmission of all of these samples (excepting pure PVAc) upon irradiation. Figs. 10 and 11 depict the transmission before and after irradition for PVAc and the 25% Kynar/75% PVAc blend. 3.5. Optical properties

Table 2 lists refractive index versus weight percent Kynar. 50% blends of Kynar 7201 with CP-41 and PVAc have refractive indices of 1.442 and 1.440, respectively. The numerical aperature (NA ) increase achieved by using these blends is calculated for three polymers using the following equation [16]: 2fs 2 NA= n -n2 , where n, and n 2 are the refractive indices of the core and cladding materials, respectively. The numerical aperature is a parameter which describes a system's ability to trap light ; higher numerical aperatures yield

sso

Table 2 Refractive indices and numerical apertures (NA ) Material

Refractive index

Pnlystyrene Polysiloxane CP-41 75/25 CP-41/Kynar 50/50 CP-41/Kynar 75/25 CP-41/Kynar

1.59 1.54 1.49 1 .46 1 .44 1.43 1.46 1.45 1.44

PVAc

75/25 PVAc/Kynar 50/50PVAc/Kynar

NA

(polystyrene) 0.00 0.56 0.62 0.67 0.71 0.63 0.66 0.67

NA

(polysilorane) 0.00 0.39 0.48 0.54 0.58 0.49 0.53 0.55

more efficient light propagation in optical fibers . This is related to the critical trapping angle in a system with two different refractive indices; the greater the difference in the indices, the larger the trapping angle. Light which impinges onto the cladding material at an angle greater than the critical angle will be scattered out of the fiber; light which impinges below the critical angle will be reflected back into the core. Consequently, light is trapped more efficiently when the difference in the core and cladding indices is large. Two representative core materials are PS (n, =1 .59) and polymethylphenylsiloxane (n, =1 .54). Calculated NA values are shown in table 2. This demonstrates that low temperature processible Kynar blends can result in a 20%-40% increase in the NA of scintillator fibers when compared with the NA of fibers made with conventional PMMA. In the case of radiation stable silox-

XAVELEf'NsTH

Fig. 11 . 0.5 mm film of 25 wt.% weight Kynar 7201 in PVAc before (

(nM)

520

»90

) andafter (---)10Mrad gammairradiation in air.

340

J. Gaynor et al. / Miscible blends offluorocarbon copolymer with acrylic copolymer and PVAc

one core fiber, a numerical aperature can be achieved which is nearly as high as the conventional PS/CP-41 combination (0.54 vs 0 .56) .

4. Conclusion Based on the data presented here, we propose the use of 50% Kynar 7201/50% PMMA blends for cladding polysiloxane scintillator fiber . This formulation is highly transparent, more radiation resistant than PMMA and results in a numerical aperature of 0 .54 with polysiloxane core material.

Acknowledgement This work was performed under U.S. Department of Energy grant DE-F605-86er40272 .

References [1] J . Harmon, J . Walker, J . Gaynor and V . Feygelman, Nucl . Instr. and Meth. B53 (1991) 300.

[2] J . Harmon,1'. Jhaveri, J . Gaynor, J. Walker and Z. Chen, ancepted by J . Appl. Poly . Sci . (1992). [3] S. Miller and A. Chenowyth (eds.), Optical Fiber Telecommunications (Academic Press, New York, 1979) p. 330 . [4] D. Wahrmund, R . Bernstein, J . Barlow and D . Paul, Poly. Eng. Sci . 18 (1978) 677. [5] R. Bernstein, D. Paul and J . Barlow, Poly. Eng . Sci. 18 (1978) 683. [6] R. Belke and 1. Cabasso, Polymer 29 (1988) 1831 . [7] R. Bernstein, D. Wahrmund, J . Barlow and D . Paul, Poly. Eng. Sci . 18 (1978) 1220. [81 J . Branchup and E. Immergut, Polymer Handbook (Wiley, New York, 1989) ch . VI, p . 221 . [9] H . Allcock and F. Lampe, Contemporary Polymer Chemistry (Prentice-Hall, New Jersey, 1981) pp. 567-580. [10] E. Collins, J . Bares and F . Billmeyer, Jr ., Experiments in Polymer Science (Wiley, New York, 1973) p . 151 . [111 D. Paul and S. Newman, Polymer Blends (Academic Press, New York, 1978) p. 147 . [12] J . Noland, N. Hsu, R. Saxon and J. Schmidt, Advances in Chemistry Series 99 (1971) 15. [131 D. Paul, J. Barlow, R . Bernstein and D. Wahlmund, Poly. Eng. Sci. 18 (1978) 1225 . [14] N . Platzer (ed.), Copolymer Polyblends and Composites, Advances in Chemistry Series 142 (1975) 371. [15] J. O'Donnell, The Effects of Radiation on High Technology Polymers Advances in Chemistry Series 381 (1989) 8 . [16] S . Miller and A . Chenowyth, op. cit. p . 346.