Article pubs.acs.org/Biomac
Spatial Glass Transition Temperature Variations in Polymer Glass: Application to a Maltodextrin−Water System Rutger M. T. van Sleeuwen,* Suying Zhang, and Valéry Normand Firmenich Inc., P.O. Box 5880, Princeton, New Jersey 08543, United States ABSTRACT: A model was developed to predict spatial glass transition temperature (Tg) distributions in glassy maltodextrin particles during transient moisture sorption. The simulation employed a numerical mass transfer model with a concentration dependent apparent diffusion coefficient (Dapp) measured using Dynamic Vapor Sorption. The mass average moisture content increase and the associated decrease in Tg were successfully modeled over time. Large spatial T g variations were predicted in the particle, resulting in a temporary broadening of the Tg region. Temperature modulated differential scanning calorimetry confirmed that the variation in Tg in nonequilibrated samples was larger than in equilibrated samples. This experimental broadening was characterized by an almost doubling of the Tg breadth compared to the start of the experiment. Upon reaching equilibrium, both the experimental and predicted Tg breadth contracted back to their initial value.
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INTRODUCTION Microencapsulation can provide protection of sensitive, volatile or reactive ingredients, while enabling controlled release of such actives.1 Commercial applications of microcapsules can be found mainly in the pharmaceutical, food, graphic arts, agrochemical, cosmetic and adhesive industries.2 Carbohydrate glasses have been used routinely to create microcapsules with extended shelf life containing volatile and unstable molecules.1 These amorphous encapsulation systems are based on reducing release or oxidation of active compounds during storage. Because these processes involve diffusion of oxygen or actives, this transfer should be minimized through proper matrix ingredient selection and control of its material properties.3 It is well-known that the diffusion or retention of volatile compounds in carbohydrate-based systems is greatly affected by the moisture content of the carrier material.4−6 Solid amorphous glasses can transform into amorphous “rubbers” by raising the temperature above the glass transition temperature (Tg). Moreover, the addition of plasticizers can have a similar effect, for example, when a carbohydrate glass gains moisture during exposure to a high relative humidity (RH). The accompanying decrease in viscosity and increase in mobility can cause individual particles or ingredients to stick together.7 Bridges between the ingredients form and, with progressive moisture uptake, this can lead to a complete structure collapse and caking of the product into a plastic mass.7,8 It is also recognized that extensive release of actives from carbohydratebased encapsulation systems occurs when the temperature is well above the Tg.9,10 To understand and control the performance and stability of carbohydrate-based glassy microcapsules, first, a good understanding of the moisture content and Tg of such systems under “equilibrium conditions” is needed. This can be accomplished by measuring the moisture sorption isotherm as well as the Tg © 2012 American Chemical Society
of the product as a function of equilibrium moisture content or water activity (Aw).11,12 Second, during processing (e.g., drying, storage or application), a microcapsule may undergo transient changes in moisture content, governed by several factors, including internal diffusion. Diffusion in glassy polymers is typically slow, although there is still significant and measurable mobility at temperatures well below Tg.13,14 This mobility within the “matrix” or “carrier” can be characterized by the apparent moisture dif f usion coef f icient (Dapp). Knowledge of this property is essential when trying to analyze a drying process.15 Often, this coefficient is strongly concentration-dependent and, to a lesser extent, temperature-dependent.16 A consequence of the relatively slow diffusion in carbohydrate glasses is that, in transient processes (e.g., moisture sorption/desorption), concentration gradients can develop even within a small particle. Bohn et al.,10 for example, noted that, during humidification, a flavor microcapsule may have a moisture gradient from surface to center and, hence, may have a gradient in Tg as well. They further mention that if the location-specific glass transition were to be measured somehow experimentally, it would differ from the spatially averaged Tg that is routinely measured by differential scanning calorimetry (DSC). The state of the surface of particles is especially significant, because all mass transfer with the surroundings has to occur through this layer. Therefore, a particle with a “depressed” surface Tg may have much greater loss of actives and enhanced mass transfer compared to a particle with uniform and “high” Tg. Also, low surface Tg can cause particles to become sticky. For example, Adhikari et al.17 note that droplets during spray Received: November 29, 2011 Revised: January 19, 2012 Published: January 23, 2012 787
dx.doi.org/10.1021/bm201708w | Biomacromolecules 2012, 13, 787−797
Biomacromolecules
Article
drying can also exhibit moisture and Tg distributions within the particle and that the surface Tg is a determining factor whether or not a droplet will stick to the dryer wall. This paper focuses on the prediction of spatial moisture content and glass transition profiles in glassy carbohydrate microcapsules during transient mass transfer processes in a dynamic vapor sorption (DVS) apparatus. This prediction is based on a numerical simulation of moisture diffusion in amorphous maltodextrin glasses. The model employs the apparent moisture diffusion coefficient and Tg, both measured as a function of moisture content. Predicted local and massaverage Tg values are compared against experimental data using DSC (temperature-modulated DSC and standard DSC). The central concept of this study is that nonequilibrated microcapsules have broader distributions of localized glass transition temperatures than equilibrated particles.
they provide sample-averaged responses without information on local variations within the sample. Nevertheless, in this study we explore the use of DSC to determine spatial glass transition temperature distributions in nonequilibrated amorphous carbohydrate particles. As will be explained below, the approach will not truly measure a localized Tg. Instead, the range of glass transitions (or Tg breadth) that exists within the sample is measured. Moisture and Tg profiles will be predicted using a numerical mass transfer simulation that divides a cylindrical microcapsule into finite ‘shells’ (see Figure 1). When such a hypothetical
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BACKGROUND To allow for experimental quantification and prediction of the ‘breadth’ of a glass transition region on equilibrated and nonequilibrated particles, a few concepts and approaches will be introduced separately. Equilibrium Tg breadth. The glass transition represents a change of a material from a disordered glassy solid with very limited mobility to a liquid melt with translational, rotational and conformational molecular motions. Calorimetrically, this transition is characterized by a step-change in heat capacity ΔC p , but without an associated heat of transition. 18 Maltodextrins typically consist of a mixture of small molecular weight oligosaccharides along with larger polysaccharides.19 Orford et al.20 showed that, for most binary mixtures of low molecular weight sugars, a linear relationship was found between Tg and the composition (mole fraction) of the mixture. Additionally, it is well-known that Tg is a function of molecular weight.21,22 Wunderlich23 described that compatible blends of homopolymers of plastics demonstrate a broadening of the glass transition region compared to the breadth of the transition of the pure homopolymers. Therefore, it is assumed here that a commercial maltodextrin, consisting of a distribution of molecular weights, would have a distribution of glass transition temperatures as well. Furthermore, it has been well established that moisture can act as a plasticizer and lower the Tg of amorphous glasses.22,24,25 Often the “Couchman−Karasz” relationship26,27 is used to predict the Tg of a mixture of compatible ingredients as a function of weight fractions and pure-compound properties of its constituents: Tg ≃
Figure 1. Schematic representation of the concept of apparent glass transition broadening in differential scanning calorimetry caused by transient spatial variations in moisture content in concentrical shells.
equilibrated particle has no spatial moisture distribution, the distribution of glass transition temperatures is mainly governed by the breadth of the distribution of the components that make up the glassy particle. Therefore, the mass or volume average Tg equals the localized Tg. However, a nonequilibrated particle with a moisture content and Tg distribution from center to surface will have regions with a Tg above and below the mass average Tg. Furthermore, spherical or cylindrical samples have more volume or mass located near the surface. Hence, the volume contribution of each “shell” is incorporated to predict the overall glass transition distribution of the whole particle. It is expected that nonequilibrated microcapsules have broader Tg distributions than equilibrated microcapsules due to water concentration gradients that exist (illustrated in Figure 1). One approach to predict this apparent Tg broadening is to combine the localized Tg, the volume of each shell and the inherent (or equilibrium) breadth of the glass transition region.
W1ΔC p1Tg1 + W2ΔC p2Tg2 W1ΔC p1 + W2ΔC p2
(1)
In eq 1, W1 and W2 are the mass fractions of components 1 and 2, respectively (e.g., water and polymer). The terms Tg1 and Tg2 represent the respective glass transition temperatures of these pure components and ΔCp1 and ΔCp2 stand for the change in heat capacity at the transition. Nonequilibrium Tg Breadth. As discussed by Gunning et al.9 and Bohn et al.,10 during moisture sorption processes in amorphous carbohydrate particles, variations in local moisture content may develop. Hence, spatial glass transition temperatures may differ from “mean” or “average” glass transition temperatures for the entire object. As Meyers et al.28 argue, thermal methods, such as DSC, suffer from the limitation that 788
dx.doi.org/10.1021/bm201708w | Biomacromolecules 2012, 13, 787−797
Biomacromolecules
Article
DRIERITE Company, LTD, Xenia, OH) was used in a desiccator to create a low relative humidity (RH) approaching 0%. All desiccators were held in a controlled-temperature room set to 25 °C. Temperature and humidity in each desiccator were recorded daily using automated data loggers (DS1923 Hygrochron, Embedded Data Systems, Lawrenceburg, KY). Vacuum was applied to each desiccator to shorten the equilibration time. The weight of a few target samples were monitored weekly until the change in mass was
