Water vapor sorption and glass transition temperatures of phase-separated amorphous blends of hydrophobically-modified starch and sucrose

This article contains water vapor sorption data obtained on amorphous blends of octenyl succinic acid-modified (denoted as hydrophobically modified starch; HMS) and sucrose (S) in the anhydrous weight HMS/S ratios between 100/0 and 27/75. The water vapor sorption data was obtained gravimetrically. The amorphous state of the blends was confirmed by X-ray diffraction. The glass transition temperatures of the phase-separated blends are listed; the blends show phase separation into a sucrose-rich phase and a HMS-rich phase, the composition of which varies with the blend ratios. The sucrose-rich phase is characterized by a glass transition temperature Tg,lower that is 40 to 90 K lower than the glass transition temperature Tg,upper of the HMS-rich phase.


Experimental features
Spray-dried blends were water activity-equilibrated at water activities 0.11, 0.

Value of the data
We present a broad set of water vapor data on blends of hydrophobically modified starch and sucrose with a systematic variation in composition. The water vapor data are obtained in the range between 0.11 and 0.75 at T ¼ 298 K.
Data on the glass transition temperatures of the phase-separated blends is valuable in the context of the understanding of the phase behavior of amorphous phase-separated systems.
These data allow the exploration of the effect of composition on water vapor sorption behavior in the glass transition range.

Data
Spray-dried blends of hydrophobically-modified starch and sucrose were water activityequilibrated at water activities 0.11, 0.22, 0.33, 0.43, 0.54, 0.68 and 0.75 (T ¼ 298 K). Water vapor sorption was determined gravimetrically until equilibrium was achieved (1200 h); the data is reported in Table 1. Water activity-equilibrated samples were analyzed for eventual crystallinity by X-ray diffraction ( Fig. 1) and for the glass transitions of the phase separated blends (sucrose-rich and modified starch-rich phases) by Differential Scanning Calorimetry (Tables 3 and 4).
The water vapor sorption data in Fig. 2 are fitted by the GAB equation (Fig. 2): where K, C and W m are fitting coefficients [3].

Experimental design, materials, and method
HMS-S blends were prepared by spray drying aqueous dispersions with well-defined ratios of HMS and S [2]. The blends were then equilibrated at a range of water activities (a w ) at T ¼ 298 K in desiccators containing saturated salt solutions (a w (salt) ¼ 0.11 (LiCl), 0.22 (CH 3 COOK), 0.33 (MgCl 2 ), 0.43 (K 2 CO 3 ), 0.54 (Mg(NO 3 ) 2 ), 0.75 (NaCl). The pure spray-dried HMS (Q 0 S ¼ 0.0) was also equilibrated at a w ¼ 0.68 (KI)). The water activities are given by Greenspan [1]. Water sorption was followed gravimetrically for 1200 h. In this time, all samples reached their equilibrium water content. The water content of the blends was determined from the weight loss/gain upon water activity equilibration and the initial water content of the blends. These initial water contents were Initial water contents of the HMS-S blends were determined by dehydration in a laboratory oven for 27 h at 253 K at a pressure below 25 mbar and under a slight flow of dry nitrogen. Powder diffraction patterns were collected using a Phillips X'pert Pro diffractometer (Panalytical) operating at 40 kV and 30 mA utilizing Cu Kα radiation (λ ¼ 0.154 nm). Scans were performed at 298 K under local atmospheric humidity over the 2θ range 5-35°with a step size of 0.02°and a data acquisition time of 2 s at each step. Glass transition temperatures were determined from the 2nd heating ramp of experiments carried out by Differential Scanning Calorimetry (DSC) as described by [2]. The midpoint glass transitions were extracted from the thermograms by deconvolution assuming the presence of multiple glass transitions each characterized by a Gaussian line shape of the first derivative of the heat flow curve [2].  Table 3 Water content and parameters associated with the glass transition fitting, as described in Section 2.4 of [2], for water activity equilibrated HMS-S blends. Q's is the weight fraction of sucrose in the HMS-S blends (on anhydrous basis), Q w is the weight fraction of water in the matrices, ΔC p,lower and ΔC p,upper are the changes in heat capacity associated with the lower and upper glass transitions, T g,lower and T g,upper are the glass transition temperatures and of the sucrose-rich and the HMS-rich phases, respectively, and ΔT g,lower and ΔT g,lower are the widths of the two glass transitions.    Table 2.