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  • 1
    Electronic Resource
    Electronic Resource
    Crystal unfolding and chain disentanglement during semicrystalline polymer dissolution (1997)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 870-876 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Mathematical models developed describe the unfolding of polymeric crystals in the presence of a solvent followed by their subsequent disentanglement. A thermodynamic model considering the free energy changes during crystal unfolding was proposed to obtain an expression for the unfolding rate. A simplified version of this expression, assuming uniform crystal size, was incorporated into a continuum model to predict the dissolution kinetics of a semicrystalline polymer slab. The model yielded predictions of the crystalline and solvent volume fractions as a function of position within the slab, in addition to changes in the fraction of polymer dissolved as well as the degree of crystallinity of the polymer as a function of time. The degree of crystallinity of the polymer decreased with time and the plot of the fraction of the polymer dissolved as a function of time exhibited Case II behavior. The model predictions agreed will with experimental results obtained during dissolution of semicrystalline poly(vinyl alcohol) in water.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690430403
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Dissolution mechanism of semicrystalline poly(vinyl alcohol) in water (1996)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 34 (1996), S. 1339-1346 
    Details
    ISSN: 0887-6266
    Keywords: poly(vinyl alcohol) ; semicrystalline ; dissolution ; differential scanning calorimetry ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Changes occurring in the degree of crystallinity and lamellar thickness distribution of poly(vinyl alcohol) (PVA) samples during dissolution in water were investigated. PVA samples of three different molecular weights were crystallized by annealing at 90, 110, and 120°C. The initial degrees of crystallinity measured by differential scanning calorimetry (DSC) and by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) varied from 43 to 60% and the average lamellar thicknesses measured by DSC ranged from 50 to 400 Å. PVA dissolution was followed at 25, 35, and 45°C from 30 s up to 195 min. Lamellar thicknesses were determined as a function of dissolution time using DSC. There was an initial drastic decrease in the degree of crystallinity, which leveled off to a fairly constant value before reaching zero by the time the polymer dissolved completely. Increase in molecular weight led to lesser number of crystals, but with larger average lamellar thickness, which were more stable in the presence of water. Increase in crystallization temperature or decrease in dissolution temperature led to larger average lamellar thickness. Based on these findings, a dissolution mechanism involving unfolding of the polymer chains of the crystal was proposed. © 1996 John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
  • 3
    Electronic Resource
    Electronic Resource
    Understanding isothermal semicrystalline polymer drying: Mathematical models and experimental characterization (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 2771-2780 
    Details
    ISSN: 0887-6266
    Keywords: drying ; semicrystalline polymers ; poly(vinyl alcohol) ; solvent removal ; modeling ; diffusion ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The drying mechanism of semicrystalline poly(vinyl alcohol) (PVA) was investigated. PVA samples of various molecular weights were crystallized by annealing at temperatures slightly above the glass transition temperature of PVA, and swollen in water for different time periods. The water volume fraction in the sample was measured using a buoyancy technique. The samples were dried in air at constant temperatures, and the drying kinetics were investigated using thermogravimetric analysis. The change in degree of crystallinity of the swollen polymer during drying was measured by differential scanning calorimetry (DSC) as well as by Fourier transform infrared spectroscopy (FTIR). The degree of crystallinity of the samples increased during drying, which in turn was found to alter the drying rate. The drying kinetics were faster at higher temperatures, for lower molecular weights, and for lower degrees of crystallinity. A mathematical model was developed to predict drying rates of semicrystalline polymers by considering the crystallization kinetics during drying. The model predictions included the thickness of the polymer sample, the degree of crystallinity of the polymer, and the water weight loss as functions of drying time. Model predictions were found to agree reasonably well with the experimental results. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2771-2780, 1998
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
  • 4
    Electronic Resource
    Electronic Resource
    Crystal dissolution-controlled release systems. II. Metronidazole release from semicrystalline poly(vinyl alcohol) systems (1997)
    Hoboken, NJ : Wiley-Blackwell
    Journal of Biomedical Materials Research 36 (1997), S. 125-130 
    Details
    ISSN: 0021-9304
    Keywords: poly(vinyl alcohol) ; controlled drug delivery ; swelling controlled-release systems ; hydrogels ; crystallinity ; dissolution ; metronidazole ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine , Technology
    Notes: Novel semicrystalline, poly(vinyl alcohol) (PVA)-based phase erosion systems were developed. The rate of drug release from these systems is controlled by the rate of crystal dissolution. PVA devices loaded with metronidazole were exposed to temperatures ranging from 90 to 120 C for times of 10-90 min to obtain samples with different degrees of crystallinity. Degrees of crystallinity were measured by differential scanning calorimetry and attenuated total reflectance Fourier transform infrared spectroscopy. In vitro release of metronidazole from such systems into deionized water at 37°C was monitored. The influence of parameters such as polymer molecular weight, annealing time and temperature, and surface pretreatment on the crystal dissolution, and hence the drug release rate, were investigated. Measurements of water-front movements were carried out to study the effect of parameters such as drug solubility on the release rate. The drug release rate was found to be dependent on the crystallization conditions of the samples. Surface pretreatment was found to reduce the burst effect observed during the release. © 1997 John Wiley & Sons, Inc. J Biomed Mater Res, 36, 125-130, 1997.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
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