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Using the Internal Stress Concept to Assess the Importance of Moisture Sorption-induced Swelling on the Moisture Transport through the Glassy HPMC Films

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Abstract

The purpose of this research was to elucidate the significance of the changes in the mechanical and the volumetric properties on the moisture diffusivity through the polymer films. The internal stress concept was adapted and applied to estimate the relative impact of these property changes on the total stress experienced by a polymer film during storage. Hydroxypropyl Methylcellulose free films were used as a model material prepared at various conditions and stored at different relative humidities. The changes in the internal stress of these films due to the moisture sorption were studied. It was demonstrated that the stress-relaxation of the films increases at increasing moisture content. At the point when there is a definite loss of stress in the film, which is at moisture content higher than 6%, was shown to correlate with the significant increase of the moisture diffusivity. Further investigations revealed that the loss of stress is especially due to the swelling of the polymer rather than the changes in the inherent strain (the quotient between the tensile strength and the modulus of elasticity) of the HPMC films. This implies that the impact of the moisture sorption on the diffusivity is predominantly via volume addition rather than via altering the mechanical properties. Additionally, the approach presented here also brings up a new application of the internal stress concept, which in essence suggests the possibility to estimate the diffusion coefficient from the sorption isotherm and the mechanical analysis data.

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Abbreviations

A :

Film area [=] m2

C avg :

Average moisture content [=] %-weight

E :

Young modulus of elasticity [=] MPa

P :

Internal stress [=] MPa

S avg :

Average solubility of moisture in the total film [=] g/m3Pa

T g :

Glass transition temperature of the coating material [=] K

V :

Original volume of the core [=] m3

WVP:

Water Vapor Permeability [=] g/m.Pa.s

Δm:

Moisture gain at certain permeation time [=] g

ΔP:

Difference of partial pressure of water vapor at two sides of the coating film [=] Pa

ΔRH:

Gradient of the relative humidity [=] %

Δt :

Time [=] s

ΔT :

Difference between the glass transition temperature of the coating and the temperature of the film during process and storage [=] K

ΔV :

Volumetric changes of the core [=] m3

Δx:

Film thickness

Δα(cubic) :

Difference in thermal expansion coefficient of the core and the coating material [=] K-1

εi :

Internal strain [=] %

εf :

Maximum strain before fracture [=] %

εtotal :

Total strain [=] %

ϕR :

Volumetric fraction of water retained in the film during storage [=] %

ϕS :

Volumetric fraction of water at solidification point (during drying) [=] %

υ:

Poisson ratio [=] -

ρtrue :

True density [=] g/m3

σ:

Stress [=] MPa

σf :

Maximum tensile strength before fracture [=] MPa

φ:

Degree of swelling [=] -

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Acknowledgements

The authors would like to thank Ramona Prickaerts, Jasper Vollenbroek, and Albert van der Padt from Friesland Food Research Centre Deventer, The Netherlands for their assistance during the measurement of the moisture sorption isotherms in their facility.

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Authors and Affiliations

  1. Department of Pharmaceutical Technology and Biopharmacy, University of Groningen, Groningen, The Netherlands

    Fesia L. Laksmana, Henderik W. Frijlink & Kees Van der Voort Maarschalk

  2. Department of Pharmaceutics, Schering–Plough Corporation, Oss, The Netherlands

    Paul J. A. Hartman Kok, Herman Vromans & Kees Van der Voort Maarschalk

  3. Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences (UIPS), Utrecht University, Utrecht, The Netherlands

    Herman Vromans

  4. PO BOX 20, Room RP2130, 5340 BH, Oss, The Netherlands

    Fesia L. Laksmana

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  1. Fesia L. Laksmana
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  2. Paul J. A. Hartman Kok
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Correspondence to Fesia L. Laksmana.

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Laksmana, F.L., Kok, P.J.A.H., Frijlink, H.W. et al. Using the Internal Stress Concept to Assess the Importance of Moisture Sorption-induced Swelling on the Moisture Transport through the Glassy HPMC Films. AAPS PharmSciTech 9, 891–898 (2008). https://doi.org/10.1208/s12249-008-9128-7

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