Sorption and Transport of Acetone in Random Copolymers of Poly(ethylene terephthalate) and Poly(ethylene 2,6-naphthalate)

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Title: Sorption and Transport of Acetone in Random Copolymers of Poly(ethylene terephthalate) and Poly(ethylene 2,6-naphthalate)
Author: McDowell, Christopher
Advisors: Benny D. Freeman, Chair
Vivian T. Stannett, Member
Carol K. Hall, Member
C. Maury Balik, Member
Abstract: Poly(ethylene terephthalate) (PET) is widely used as a barrier packagingmaterial for several food and beverage products. However, PET does not havesufficient barrier properties for many food and beverage packaging applications.The shelf life of these products can be compromised by the migration ofsmall molecules such as oxygen, water, or organic flavor molecules throughpackaging materials. Copolymerization of PET with poly(ethylene 2,6-naphthalate)(PEN) leads to polymers with a better barrier properties. The focus of thisdissertation is to elucidate the fundamental sorption and transport propertiesof small molecules in random copolymers of PET and PEN in order to developsystematic structure/property relations for this family of polymers. The sorption and kinetics of acetone uptake in solvent cast films ofpoly(ethylene terephthalate) are reported at 35° C for acetone pressuresranging from 0 to 7.3 cm Hg. The equilibrium sorption isotherm is well described by the dual-mode sorption model with the following parameters: k= 61 cm,and b = 50 atm-1. Sorption kinetics are described using a two-stage modelwhich incorporates both Fickian diffusion and protracted polymer structuralrelaxation. The characteristic time associated with the relaxation processis essentially independent of acetone concentration and has an average valueof approximately 15 hours. The fraction of sorption associated with polymerrelaxation increases linearly with acetone concentration in the equilibriumdensified matrix of the polymer. Acetone diffusion coefficients increasewith increasing acetone concentration. The concentration dependence of theacetone diffusion coefficient is well described by the dual-mobility modelif the assumption of constant diffusion coefficients in the two modes isrelaxed. Random copolymers of poly(ethylene terephthalate) [PET] and poly(ethylene2,6-naphthalate) [PEN] were synthesized by melt condensation. The glass/rubbertransition temperatures of these materials rise monotonically with increasingPEN content. In a series of thin, solvent cast films of varying PEN content,acetone diffusivity and solubility were determined at 35° C and an acetonepressure of 5.4 cm Hg. The kinetics of acetone sorption in the copolymerfilms are well described by a Fickian model. Both solubility and diffusivitydecrease with increasing PEN content. The acetone diffusion coefficientdecreases 93% from PET to PET/85PEN. The acronym PET/85PEN refers to a polymerin which 85 weight percent of the dimethyl terephthalate in PET has beenreplace by dimethyl naphthalate 2,6-dicarboxylate. The acetone solubilitycoefficient in the amorphous regions of the polymer decreases by approximatelya factor of two over the same composition range. Copolymers containing between20 and 80 wt. % PEN are amorphous. Samples with less than 20% or more than80% PEN contain measurable levels of crystallinity. Estimated fractionalfree volume in the amorphous regions of these samples is lower in the copolymersthan in either of the homopolymers. Relative free volume as probed by positronannihilation lifetime spectroscopy [PALS] decreases systematically withincreasing PEN content. Acetone diffusion coefficients correlate well withPALS results. Infrared spectroscopy suggests an increase in the fractionof ethylene glycol units in the trans conformation in the amorphous phaseas the concentration of PEN in the copolymer increases. The interval sorption kinetics of acetone in solvent cast films of randompoly(ethylene terephthalate)-co-(ethylene 2,6-naphthalate) are reportedat 35° C and at acetone pressures ranging from 0 to 7.3 cm Hg. Equilibriumsorption is well described by the dual-mode sorption model. Interval sorptionkinetics are described using a two-stage model, which incorporates bothFickian diffusion and protracted polymer structural relaxation. The incorporationof low levels of PEN into PET appears to reduce the excess free volume associatedwith the glassy state and, for these interval acetone sorption experimentsin ~5μm thick films, decreases the fraction of acetone uptake controlledby penetrant-induced polymer structural relaxation. A systematic series of copolyesters was prepared by replacing up to 10weight percent of the dimethyl terephthalate [DMT] in poly(ethylene terephthalate)[PET] with dimethyl 2,6-naphthalene dicarboxylate [NDC], isophthalic acid[IPA], or 2,5-bis-(4-carboxyphenyl)-1,3,4-oxadiazole [ODCA]. Solution castfilms of the resulting copolymers were prepared and characterized. Modificationof PET with NDC and ODCA led to copolymers with glass transition temperatureshigher than that of PET, while modification with IPA decreased the glasstransition temperature. Copolymerization decreased crystallinity in allcases. The acetone solubility and acetone diffusion coefficient were determinedby integral kinetic gravimetric sorption experiments at 35° C and 5.4cm Hg acetone pressure. PET containing low levels of NDC had lower acetonediffusivity and lower acetone solubility than PET, while PET modified withIPA had higher acetone diffusivity and lower acetone solubility than PET.PET modified with 5% ODCA had higher acetone diffusivity, while PET modifiedwith 10% ODCA had an acetone diffusivity value similar to that of PET. PETcopolymers containing ODCA had higher acetone solubilities that PET.
Date: 1998-04-13
Degree: PhD
Discipline: Chemical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/5457


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