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One cause of long-term dimensional changes in glassy polymers is the gradual evolution of the glassy structure and viscoelastic properties through aging processes in the glassy state. Many applications of polymers involve changes in relative humidity (RH), under which the materials exhibit aging processes that may differ from those in constant RH conditions. In this work, results from a study of a glassy epoxy subjected to isothermal RH-jumps are reported. Similar to the temperature jump experiments of Kovacs, the volume recovery responses in different histories, intrinsic isopiestic (constant RH), memory effect and the asymmetry of approach, are obtained.

In addition, the effects of structural recovery on sorption and desorption of water and the physical aging responses of this epoxy resin are also measured. The experimental results qualitatively support the hypothesis that water has an effect similar to temperature on structure (volume, enthalpy) of the glass-forming material.

However, quantitatively, at the same RH-temperature states, the glasses formed by RH jumps show anomalous differences from those formed by temperature jumps.

The conventional TNM-KAHR model is modified to fit the experimental results of isothermal RH jump experiments. Furthermore, application of the potential energy landscape theory to describe the glasses formed by relative humidity jumps, and especially to the observed differences between temperature glass and concentration glass, is discussed.

Content:

I. INTRODUCTION

1.1 Glass transition and structural recovery
1.2 Depression of the glass transition temperature
1.3 Effects of small molecules on the viscoelastic properties
1.4 Structural recovery introduced by sorption and desorption of plasticizers
1.5 Explanation of moisture-induced structural change
1.6 Evidence of water or other small molecules inducing enthalpy relaxation
1.7 Impact of voids on the equilibrium mass uptake and sorption kinetics
1.8 State of water in the epoxy network
1.9 Fundamental hypothesis

II. EXPERIMENTAL SYSTEM

2.1 Environmental chamber
2.2 Volume measurements
2.3 Creep measurements
2.4 Measurement of mass uptake of water
2.5 Sample preparation

III. EXPERIMENTAL RESULTS

3.1 Structural recovery after RH-jump
3.1.1 Structural recovery experiments
3.1.2 Volume change during isothermal relative humidity down-jump
3.1.3 Intrinsic isopiestics
3.1.4 Asymmetry of approach
3.1.5 Memory experiment
3.1.6 Relative humidity change versus temperature change

3.2 Mass uptake of water

3.2.1 Transition in mass uptake experiments
3.2.2 Isopiestics in mass uptake experiments
3.2.3 Asymmetry of approach in mass uptake experiments
3.2.4 Memory effects in mass uptake experiments

3.3 Physical aging subsequent to relative humidity jumps

3.3.1 Physical aging experiments
3.3.2 Method of analysis
3.3.3 Time-relative humidity (RH) superposition
3.3.4 Creep responses during isopiestic structural recovery
3.3.5 Creep response in memory experiments
3.3.6 Creep responses in asymmetry of approach experiments
3.3.7 Comparison between creep compUances of T-jump and RHjump
experiments

IV. NON-LINEAR TNM-KAHR MODEL

4.1 Introduction to TNM-KAHR model
4.2 Determining nonUnear parameter x from experimental data
4.2.1 Introduction
4.2.2 Simulation of the DSC data
4.2.3 Analysis of simulated experiments
4.2.4 Results
4.2.5 Discussion

4.3 Extension of KAHR model to RH-jump conditions

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